[StaticWebAssets] Fix duplicate asset detection in GroupAssetsByTargetPath

[javiercn]

Summary

Fixes a bug where GenerateStaticWebAssetsManifest fails to detect conflicting static web assets when they have different RelativePath patterns that resolve to the same target path after token replacement.

Problem

When a file in wwwroot matches the naming pattern of a scoped CSS bundle (e.g., MyApp.styles.css), the build fails with:

System.InvalidOperationException: Sequence contains more than one element
   at System.Linq.ThrowHelper.ThrowMoreThanOneElementException()
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.GenerateStaticWebAssetsDevelopmentManifest.ComputeManifestAssets()

The root cause is that GroupAssetsByTargetPath used the 2-argument ComputeTargetPath("", '/') which keeps fingerprint tokens in the path, while GenerateStaticWebAssetsDevelopmentManifest uses the 3-argument version with StaticWebAssetTokenResolver.Instance which replaces tokens.

Fix

Use StaticWebAssetTokenResolver.Instance in GroupAssetsByTargetPath to ensure consistent conflict detection.

Testing

• Added unit tests in StaticWebAssetTest.cs to verify the fix
• All existing GenerateStaticWebAssetsManifestTest tests pass

[dotnet-policy-service]

Thanks for your PR, @@javiercn.
To learn about the PR process and branching schedule of this repo, please take a look at the SDK PR Guide.

[javiercn]

Came as a side-effect of investigating #50183

[Copilot]

Pull request overview

This PR fixes a bug in GenerateStaticWebAssetsManifest where duplicate asset detection failed when assets had different RelativePath patterns that resolved to the same target path after token replacement. The fix ensures consistent token resolution by using StaticWebAssetTokenResolver.Instance in the GroupAssetsByTargetPath method.

Key Changes

• Updated GroupAssetsByTargetPath to use the 3-argument ComputeTargetPath overload with token resolver for consistent duplicate detection
• Added comprehensive unit tests verifying token replacement behavior and conflict detection across different asset patterns

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsManifest.cs	Fixed GroupAssetsByTargetPath to use StaticWebAssetTokenResolver.Instance for consistent target path computation
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/StaticWebAssetTest.cs	Added comprehensive unit tests covering token replacement behavior, conflict detection, and validation of assets with different patterns resolving to the same path