Fix StaticWebAssets base path prefix detection using path segment comparison

[javiercn]

Summary

When computing endpoints for reference static web assets, the base path prefix check was using string.StartsWith which incorrectly matched routes like App1.styles.css against base path App1. This caused the base path to not be prepended, resulting in missing routes.

Problem

The bug occurs in ComputeEndpointsForReferenceStaticWebAssets when determining if a route already has the base path applied. The original check:

if (oldRoute.StartsWith(asset.BasePath, StringComparison.Ordinal))

Would incorrectly match:

• Route App1.styles.css with base path App1 → incorrectly skipped (starts with "App1")
• Route App1.lib.module.js with base path App1 → incorrectly skipped

This resulted in these files being served without the proper base path prefix in multi-project Blazor scenarios.

Solution

Introduced StaticWebAssetEndpoint.RouteHasPathPrefix method that uses PathTokenizer to compare paths segment-by-segment, ensuring only proper path segment boundaries are matched:

Route	Prefix	Result	Reason
App1/css/app.css	App1	✅ true	Proper segment match
App1.styles.css	App1	❌ false	Not a segment boundary
App1Bundle/app.js	App1	❌ false	Different segment
App1	App1	✅ true	Exact match

The comparison is case-insensitive and handles both / and \ path separators.

Testing

• Added unit tests for RouteHasPathPrefix covering positive and negative matches
• Added regression tests in ComputeEndpointsForReferenceStaticWebAssetsTest
• Validated E2E with multi-project Blazor WebAssembly app (App1, App2, AppHost)

Files Changed

• ComputeEndpointsForReferenceStaticWebAssets.cs - Use new segment-based prefix check
• StaticWebAssetEndpoint.cs - Add RouteHasPathPrefix helper method
• StaticWebAssetEndpointTest.cs - Unit tests for RouteHasPathPrefix
• ComputeEndpointsForReferenceStaticWebAssetsTest.cs - Regression tests

[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.

[Copilot]

Pull request overview

This PR fixes a bug in static web assets base path prefix detection where string.StartsWith incorrectly matched routes like App1.styles.css against base path App1, causing missing route prefixes in multi-project Blazor scenarios. The solution introduces segment-based path comparison using PathTokenizer to ensure only proper path segment boundaries are matched.

Key Changes:

• Introduced StaticWebAssetEndpoint.RouteHasPathPrefix for segment-based path prefix comparison
• Updated ComputeEndpointsForReferenceStaticWebAssets to use the new method instead of string.StartsWith
• Added comprehensive unit tests and regression tests

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/StaticWebAssetsSdk/Tasks/Data/StaticWebAssetEndpoint.cs	Added RouteHasPathPrefix static method for segment-based path prefix comparison using PathTokenizer
src/StaticWebAssetsSdk/Tasks/ComputeEndpointsForReferenceStaticWebAssets.cs	Replaced string.StartsWith with RouteHasPathPrefix and added segment list reuse for performance
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/StaticWebAssetEndpointTest.cs	New test file with comprehensive unit tests for RouteHasPathPrefix covering positive/negative matches and edge cases
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ComputeEndpointsForReferenceStaticWebAssetsTest.cs	Added regression tests validating the bug fix and correct base path application behavior