fix(stryker): add Microsoft Designer test to tap.testFiles

[Subbu1399]

Summary

Adds the missing mutation coverage test file to tap.testFiles in stryker.conf.json so the mutation coverage check includes the test.

Related Issues

• Closes test(ci): register microsoft-designer-web-6672 in stryker tap.testFiles #7657

Validation

• npm run lint (not run)
• npm run test:unit (not run)
• npm run test:coverage (not run)
• Coverage is still >= 60% for statements, lines, functions, and branches (not run)
• SonarQube PR analysis is green or any remaining issues are explicitly documented below (pending CI)

Additional validation:

• ✅ node scripts/check/check-mutation-test-coverage.mjs --strict

Tests Added Or Updated

• Updated stryker.conf.json to include:
  • tests/unit/microsoft-designer-web-6672.test.ts
• No test files were modified.

Coverage Notes

No production code (src/, open-sse/, electron/, or bin/) was changed. This PR only updates the mutation test configuration.

Reviewer Notes

Low-risk configuration change. Added the missing test entry in alphabetical order.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[Subbu1399]

Fixes #7657 by adding the missing mutation coverage test entry to tap.testFiles in stryker.conf.json.