[StaticWebAssets] Fix recursive glob stem truncation for multi‑dot component JS module files and expand pattern coverage

[javiercn]

Description

Recursive glob patterns like **/*.razor.js and **/*.cshtml.js produced an incorrect relative path for files whose base name contained multiple dots.

For example, for a pattern like:
Pattern: **/*.razor.js
Path: Components/User.Profile.Details.razor.js
Stem (incorrect): User.Profile.Details.razor.js (directory portion lost)

Fixes #50840

Customer Impact

Without this fix, any file with multiple extensions that is filtered during build time processing receives incorrect relative paths which translates in harder to debug issues later during build or at runtime. For example, the file will be served from the wrong file path during runtime.

Regression?

• Yes
• [] No

9.0

Risk

• High
• Medium
• Low

Justification: Change is localized to preserving a single integer
(StemStartIndex) across extension matching. No algorithmic branching
added; existing broad test suite plus new targeted cases all pass.

Verification

• Manual
• Automated

Added tests for the problematic cases and additional tests for variations of the pattern.

Packaging changes reviewed?

• Yes
• No
• N/A

[javiercn]

/backport to main

[github-actions]

Started backporting to main: https://github.com/dotnet/sdk/actions/runs/17767118227

[baronfel]

@javiercn you shouldn't need to explicitly backport to main - we have automation to merge release/10.0.1xx forward to main.

[javiercn]

@baronfel I thought that wasn't the case. I'll close the backport then.

[baronfel]

No worries, just hoping to save you some work. The codeflow definitions are kept in this repo at

https://github.com/dotnet/sdk/blob/main/github-merge-flow.jsonc#L29

if you ever need to reference them in the future.