[StaticWebAssets] Compress generated html file

[maraf]

• Remove OriginalItemSpec from generated html asset. It results in RelatedAssetOriginalItemSpec not being set and thus correct RelatedAsset is used when deciding which file to compress https://github.com/dotnet/sdk/blob/main/src/StaticWebAssetsSdk/Tasks/Utils/AssetToCompress.cs#L15
• Read and assert gzip and brotli compressed html assets in test

Fixes dotnet/aspnetcore#61937

[Copilot]

Pull Request Overview

This PR refines the Static Web Assets compression behavior by excluding the OriginalItemSpec metadata from generated HTML asset items and adding tests to verify gzip and brotli compressed output.

• Removed OriginalItemSpec metadata for HTML assets to ensure the correct RelatedAsset is used.
• Extended tests to read and assert both gzip and brotli compressed HTML files.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
test/Microsoft.NET.Sdk.Razor.Tests/StaticWebAssetsFingerprintingTest.cs	Added assertions to verify decompressed HTML content for gzip and brotli files and included required using statement for compression.
src/StaticWebAssetsSdk/Tasks/OverrideHtmlAssetPlaceholders.cs	Updated task code to remove the OriginalItemSpec metadata from generated HTML asset items.