fix(linter/no-unused-vars): allow unused type params in ambient module blocks

[DonIsaac]

What This PR Does

Fixes a bug in eslint/no-unused-vars where type parameters used in merged interfaces were marked as unused.

Example

declare module 'bun:test' {
  // `T` is marked as unused, but removing it causes a type error.
  // This PR flags this param as allowed.
  interface Matchers<T> {
    toBeFoo(value: unknown): unknown;
  }
}

[Copilot]

Pull request overview

This PR fixes a bug in the eslint/no-unused-vars rule where type parameters used in merged interfaces within ambient module blocks (e.g., declare module 'bun:test') were incorrectly marked as unused. Type parameters in such contexts are required for declaration merging to work, as signatures must match.

Changes:

• Added logic to allow unused type parameters in ambient TypeScript module blocks
• Added test cases for both ambient (declare module) and non-ambient (namespace) scenarios
• Updated snapshot to reflect the new test case for non-ambient namespaces

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/oxc_linter/src/rules/eslint/no_unused_vars/allowed.rs	Added logic to check if a type parameter is within an ambient TS module block and allow it if the module declaration has declare = true
crates/oxc_linter/src/rules/eslint/no_unused_vars/tests/oxc.rs	Added pass test for ambient module with unused type parameter and fail test for non-ambient namespace with unused type parameter
crates/oxc_linter/src/snapshots/eslint_no_unused_vars@oxc-namespaces.snap	Updated snapshot to include the expected diagnostic for the new fail test case

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks
⏩ 52 skipped benchmarks¹

---

_{Comparing DonIsaac:don/fix/nuv/ty-param-decl-merging (0af9fce) with main (eea201c)²}

Footnotes

1. 52 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. ↩

2. No successful run was found on main (4bf383d) during the generation of this report, so eea201c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[camc314]

Thank you Don!