fix: Auto-update package.json FileVersion when downloading new spec files

[jtgreen86]

Summary

Fixes an oversight in the spec file update workflow where new Splunk versions were downloaded to spec_files/ but not added to the splunk.spec.FileVersion setting in package.json. Users had to manually update the extension settings to use newly downloaded spec files.

Changes

• Add updatePackageJsonVersions() function to scripts/update-spec-files.js that automatically:
  • Adds new versions to the splunk.spec.FileVersion enum
  • Sets the newest version as the default
• Integrate the update into the existing workflow (Step 5)
• Update PR report to include package.json changes
• Add documentation to .github/copilot-instructions.md
• Update package.json with missing versions: 9.3, 9.4, 10.0, 10.2, 10.4

Test Plan

• Run node scripts/update-spec-files.js --check-only --verbose to verify script parses correctly
• Verify package.json enum matches spec_files/ directories
• Run npm test to ensure no regressions

Note: The functional-test macOS CI failure is a pre-existing issue on
master unrelated to this PR. VS Code 1.133.0 downloads successfully but the
Electron binary fails to spawn on the macos-latest ARM runner (ENOENT).
Unit tests pass on all platforms.

[JasonConger]

We updated the actions to use v7 recently. The MR uses v4 in build-test.yaml. Please test with v7. I don't anticipate any issues, but let's check.