ci: use standard arm64 runners for pull request benchmarks

[hellobertrand]

Update the benchmark workflow to use the standard 'macos-26' runner for pull requests, while retaining the 'macos-26-xlarge' runner for push and dispatch events. This reduces resource consumption during routine PR testing while maintaining performance capacity for main branch benchmarks.

Summary by CodeRabbit

• Chores
  • Benchmark workflows now use a different macOS runner for pull requests and other events; Ubuntu benchmarks continue to use the latest Ubuntu runner.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 6d742840-9c9c-41b5-a9d5-c4c6de04fc49

📥 Commits

Reviewing files that changed from the base of the PR and between b9204c6 and 9798087.

📒 Files selected for processing: 1

• .github/workflows/benchmark.yml

Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 3 remain after this review.

📝 Walkthrough

The benchmark workflow keeps Ubuntu in its OS matrix. It selects macos-26 for pull requests and macos-26-xlarge for other events.

Changes: Benchmark runner selection

Layer / File(s)	Summary
Event-based OS matrix .github/workflows/benchmark.yml	The benchmark OS matrix retains Ubuntu and selects macos-26 for pull requests or macos-26-xlarge for other events.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to 97980

Pull-request benchmarks use the documented standard macOS runner while push and dispatch benchmarks retain the xlarge runner. The matrix is valid, with no identified issue blocking merge.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly describes the main change: using standard runners for pull request benchmarks. It is concise and specific.
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

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