fix(linter): copy over js plugins when applying overrides

[lilnasy]

• quick fix. closes linter: overrides disables JS plugins #14504
• added repro as test
  • fails without the fix
  • passes with it
• Direct edits from maintainers welcome.

[graphite-app]

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[Copilot]

Pull Request Overview

This PR fixes a bug where external JavaScript plugin rules were being lost when applying ESLint config overrides. The issue was that the code was creating an empty external_rules map instead of copying over the base external rules when processing overrides.

• Fixed initialization of external_rules to preserve base external rules when applying overrides
• Added a comprehensive test case that reproduces the bug and verifies the fix

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[codspeed-hq]

CodSpeed Performance Report

Merging #14542 will not alter performance

_{Comparing lilnasy:fix/14504 (b70385b) with main (9cd7a67)}

Summary

✅ 4 untouched
⏩ 33 skipped¹

Footnotes

1. 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[camc314]

thank you!