Refactor RequestChecksumInterceptor into distinct interceptors (#4384)

## Motivation and Context
This PR breaks up `RequestChecksumInterceptor` into two interceptors:
`RequestChecksumInterceptor` and `AwsChunkedContentEncodingInterceptor`
with additional code restructuring to support the upcoming aws-chunked
content encoding enhancement.

## Description
`RequestChecksumInterceptor` previously handled two distinct
responsibilities:
1. Adding checksums (body or trailer)
2. Applying aws-chunked content encoding

This refactor extracts the second responsibility (aws-chunked logic)
into `AwsChunkedContentEncodingInterceptor`, aiming to enable it via the
dedicated trait when available in Smithy.

In addition, both interceptors now wrap bodies in
`modify_before_transmit` instead of `modify_before_signing` per design
requirements.

## Tips for merging to `feature/http-1.x`

This refactor reluctantly adds `http_0x` constructs that will conflict
with `feature/http-1.x`. Update these files when merging:
- `aws-inlineable/src/http_request_checksum.rs` - Apply same 1.x updates
as done [in
branch](https://github.com/smithy-lang/smithy-rs/compare/feature/http-1.x?expand=1#diff-7c4006e142da69a7a442cd1d152bf5d0ae05a18c93a02c9754776ef29cef5dcd)
(note: `test_checksum_body_is_retryable` was removed as it tested a
utility only used in streaming cases against non-streaming bodies)
- `aws-inlineable/src/aws_chunked.rs` - Use 1.x for header names,
values, and body trait methods in unit tests, as in
`http_request_checksum.rs`
- `aws/rust-runtime/aws-runtime/src/content_encoding.rs` - Needs 1.x
updates
```
    fn size_hint(&self) -> http_body_1x::SizeHint {
        http_body_1x::SizeHint::with_exact(self.encoded_length())
    }
```
to
```
    fn size_hint(&self) -> http_body_1x::SizeHint {
        http_body_1x::SizeHint::with_exact(self.options.encoded_length())
    }
```
because the `.encoded_length()` method has been moved from
`AwsChunkedBody` to `AwsChunkedBodyOptions` (due to the `content-length`
header needing encoded length during signing but body creation deferred
to
`modify_before_transmit`)

## Testing
- CI

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Author: ysaito1001

aws/codegen-aws-sdk/src/main/kotlin/software/amazon/smithy/rustsdk/AwsChunkedContentEncodingDecorator.kt

@@ -0,0 +1,93 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.rustsdk
+
+import software.amazon.smithy.aws.traits.HttpChecksumTrait
+import software.amazon.smithy.model.shapes.OperationShape
+import software.amazon.smithy.model.shapes.StructureShape
+import software.amazon.smithy.model.traits.HttpHeaderTrait
+import software.amazon.smithy.rust.codegen.client.smithy.ClientCodegenContext
+import software.amazon.smithy.rust.codegen.client.smithy.customize.ClientCodegenDecorator
+import software.amazon.smithy.rust.codegen.client.smithy.generators.OperationCustomization
+import software.amazon.smithy.rust.codegen.client.smithy.generators.OperationSection
+import software.amazon.smithy.rust.codegen.core.rustlang.CargoDependency
+import software.amazon.smithy.rust.codegen.core.rustlang.Visibility
+import software.amazon.smithy.rust.codegen.core.rustlang.rustTemplate
+import software.amazon.smithy.rust.codegen.core.rustlang.writable
+import software.amazon.smithy.rust.codegen.core.smithy.RuntimeConfig
+import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType
+import software.amazon.smithy.rust.codegen.core.util.getTrait
+import software.amazon.smithy.rust.codegen.core.util.hasStreamingMember
+
+class AwsChunkedContentEncodingDecorator : ClientCodegenDecorator {
+    override val name: String = "AwsChunkedContentEncoding"
+
+    // This decorator must decorate after any of the following:
+    // - HttpRequestChecksumDecorator
+    // - HttpRequestCompressionDecorator
+    override val order: Byte = (minOf(HttpRequestChecksumDecorator.ORDER, HttpRequestCompressionDecorator.ORDER) - 1).toByte()
+
+    override fun operationCustomizations(
+        codegenContext: ClientCodegenContext,
+        operation: OperationShape,
+        baseCustomizations: List<OperationCustomization>,
+    ) = baseCustomizations + AwsChunkedOparationCustomization(codegenContext, operation)
+}
+
+private class AwsChunkedOparationCustomization(
+    private val codegenContext: ClientCodegenContext,
+    private val operation: OperationShape,
+) : OperationCustomization() {
+    private val model = codegenContext.model
+    private val runtimeConfig = codegenContext.runtimeConfig
+
+    override fun section(section: OperationSection) =
+        writable {
+            when (section) {
+                is OperationSection.AdditionalInterceptors -> {
+                    // TODO(https://github.com/smithy-lang/smithy-rs/issues/4382): Remove all of these early returns
+                    //  once we have the dedicated trait available in Smithy.
+                    val checksumTrait = operation.getTrait<HttpChecksumTrait>() ?: return@writable
+                    val requestAlgorithmMember =
+                        checksumTrait.requestAlgorithmMemberShape(codegenContext, operation) ?: return@writable
+                    requestAlgorithmMember.getTrait<HttpHeaderTrait>()?.value ?: return@writable
+                    val input = model.expectShape(operation.inputShape, StructureShape::class.java)
+                    if (!input.hasStreamingMember(model)) {
+                        return@writable
+                    }
+
+                    section.registerInterceptor(runtimeConfig, this) {
+                        rustTemplate(
+                            """
+                            #{AwsChunkedContentEncodingInterceptor}
+                            """,
+                            "AwsChunkedContentEncodingInterceptor" to
+                                runtimeConfig.awsChunked()
+                                    .resolve("AwsChunkedContentEncodingInterceptor"),
+                        )
+                    }
+                }
+
+                else -> emptySection
+            }
+        }
+}
+
+private fun RuntimeConfig.awsChunked() =
+    RuntimeType.forInlineDependency(
+        InlineAwsDependency.forRustFile(
+            "aws_chunked", visibility = Visibility.PUBCRATE,
+            CargoDependency.Bytes,
+            CargoDependency.Http,
+            CargoDependency.HttpBody,
+            CargoDependency.Tracing,
+            AwsCargoDependency.awsRuntime(this).withFeature("http-02x"),
+            CargoDependency.smithyRuntimeApiClient(this),
+            CargoDependency.smithyTypes(this),
+            AwsCargoDependency.awsSigv4(this),
+            CargoDependency.TempFile.toDevDependency(),
+        ),
+    )

aws/codegen-aws-sdk/src/main/kotlin/software/amazon/smithy/rustsdk/AwsCodegenDecorator.kt

@@ -56,6 +56,7 @@ val DECORATORS: List<ClientCodegenDecorator> =
             SdkConfigDecorator(),
             ServiceConfigDecorator(),
             AwsPresigningDecorator(),
+            AwsChunkedContentEncodingDecorator(),
             AwsCrateDocsDecorator(),
             AwsEndpointsStdLib(),
             *PromotedBuiltInsDecorators,

aws/codegen-aws-sdk/src/main/kotlin/software/amazon/smithy/rustsdk/HttpRequestChecksumDecorator.kt

@@ -55,8 +55,12 @@ internal fun RuntimeConfig.awsInlineableHttpRequestChecksum() =
     )
 
 class HttpRequestChecksumDecorator : ClientCodegenDecorator {
+    companion object {
+        const val ORDER: Byte = 0
+    }
+
     override val name: String = "HttpRequestChecksum"
-    override val order: Byte = 0
+    override val order: Byte = ORDER
 
     override fun operationCustomizations(
         codegenContext: ClientCodegenContext,

aws/codegen-aws-sdk/src/main/kotlin/software/amazon/smithy/rustsdk/HttpRequestCompressionDecorator.kt

@@ -21,8 +21,12 @@ import software.amazon.smithy.rust.codegen.core.smithy.customize.adhocCustomizat
 import software.amazon.smithy.rust.codegen.core.util.thenSingletonListOf
 
 class HttpRequestCompressionDecorator : ClientCodegenDecorator {
+    companion object {
+        const val ORDER: Byte = 0
+    }
+
     override val name: String = "HttpRequestCompression"
-    override val order: Byte = 0
+    override val order: Byte = ORDER
 
     private fun usesRequestCompression(codegenContext: ClientCodegenContext): Boolean {
         val index = TopDownIndex.of(codegenContext.model)