[Static Web Assets] Fix relative path matching

[javiercn]

When discovering assets we also need to account for the case where the asset was provided with a full path (rare but happens) and make the path relative against the project folder before we process it.

Fixes #49038

[dotnet-policy-service]

Thanks for your PR, @javiercn.
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[Copilot]

Pull Request Overview

This PR fixes relative path matching for static web assets when they are provided with full paths during asset discovery. The fix ensures that full paths are properly converted to relative paths before processing, which is necessary for correct asset handling.

• Adds logic to detect when a candidate asset path is rooted and converts it to a relative path
• Includes comprehensive test coverage for the new functionality across different scenarios
• Handles both Windows and Linux path formats appropriately

Reviewed Changes

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

File	Description
src/StaticWebAssetsSdk/Tasks/DefineStaticWebAssets.cs	Adds path normalization logic to convert full paths to relative paths when discovering assets
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/DiscoverStaticWebAssetsTest.cs	Adds three new test methods to verify relative path computation for different scenarios