feat(azdext): Add KeyVaultResolver to Extension SDK for Key Vault secret resolution

[jongio]

Summary

Add a KeyVaultResolver to the Extension SDK (pkg/azdext) that resolves Azure Key Vault secret references embedded in environment variables. This eliminates ~100+ lines of duplicated Key Vault resolution logic across each extension.

Motivation

Extensions running scripts or managing environments need to resolve Azure Key Vault references embedded in environment variables. Without framework support, each extension imports duplicated infrastructure:

• azd-exec uses Key Vault resolution with a StopOnKeyVaultError config flag and factory pattern
• azd-app implements custom Config/AppConfig structs with Load(), Save(), and AtomicWriteJSON
• azd-core provides shared keyvault package that every extension depends on

Evidence: azd-exec KV resolution, azd-core keyvault package

Supported Reference Formats

• akvs://<subscription>/<vault>/<secret>
• @Microsoft.KeyVault(SecretUri=https://<vault>.vault.azure.net/secrets/<name>[/<version>])
• @Microsoft.KeyVault(VaultName=...;SecretName=...)

Features

• KeyVaultResolver: Thread-safe per-vault client caching, batch resolution via ResolveMap, structured error types with ResolveReason classification, configurable vault suffix for sovereign clouds
• Helper functions: IsSecretReference, ParseKeyVaultAppReference, ResolveSecretEnvironment for bulk env var resolution
• Integration point: cmd/extensions.go calls ResolveSecretEnvironment before passing env vars to extensions, so extensions receive plain secret values transparently
• Core keyvault additions: IsKeyVaultAppReference, ParseKeyVaultAppReference, SecretFromKeyVaultReference, ResolveSecretEnvironment in pkg/keyvault

Files

• cli/azd/pkg/azdext/keyvault_resolver.go — KeyVaultResolver with Resolve, ResolveMap, error types
• cli/azd/pkg/azdext/keyvault_resolver_test.go — comprehensive tests (577 lines)
• cli/azd/pkg/keyvault/keyvault.go — helper functions for @Microsoft.KeyVault format parsing
• cli/azd/cmd/extensions.go — integration: resolve KV refs before passing env to extensions
• cli/azd/internal/cmd/show/show.go — documentation comments for KV resolution

Related

• Part of #6853 Extension Framework Improvements
• Originally in #6945 P1 Integration Helpers (bundled with Config Helpers)
• Extracted from PR #7025 to reduce PR scope

Branch

feature/ext-sdk-kv-resolver on jongio/azure-dev