fix(module-runner): prevent crash when sourceMappingURL pattern appears in string literals

[Copilot]

Fixes #20551

Problem

The ModuleRunner was crashing with InvalidCharacterError: Invalid character when source code contained string literals that included the sourceMappingURL pattern. This happened because the regex would match the first occurrence of the pattern instead of the last one, potentially picking up invalid base64 data from string literals.

// This code would crash the ModuleRunner
const text = "//# sourceMappingURL=data:application/json;base64,invalidbase64";
// ... rest of code ...
//# sourceMappingURL=data:application/json;base64,validBase64Here

The original regex would match the invalid base64 from the string literal, causing atob() to throw when trying to decode it.

Solution

Updated the approach to find the last occurrence of the sourceMappingURL pattern:

1. Hoisted regex: Kept the original MODULE_RUNNER_SOURCEMAPPING_REGEXP with (.+) pattern for performance
2. Last occurrence detection: Use lastIndexOf to find the last occurrence of the sourceMappingURL pattern and process only that line
3. Graceful error handling: Added try-catch to handle invalid base64 or malformed JSON gracefully

Testing

Added test case in server-source-maps.spec.ts that verifies:

• ✅ No crash when sourceMappingURL appears in string literals
• ✅ Correct extraction of valid sourcemaps at end of files
• ✅ All existing functionality remains intact

Impact

This is a targeted fix that resolves the crash without affecting any existing functionality. The approach now correctly identifies the actual sourcemap comment at the end of files while ignoring similar patterns that appear in string literals.

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