Migrate Mocha and test configuration to v12

[Copilot]

Upgrade Mocha to 12.0.2 using npm 11.19 with --min-release-age 7.

• Configuration: Update debugger entrypoints and setup-module paths; load ambient declarations for TypeScript test runs.
• Reporting: Replace incompatible multi/JUnit reporter dependencies with Mocha’s built-in Spec/XUnit reporters, preserving PID-prefixed console output and MOCHA_FILE.
• Completion: Flush XML before notifying the extension-test socket server, preventing truncated reports.
• Regression coverage: Cover passing, failing, and pending XML results, output-path precedence, and extension-test completion.