Add --dry-run flag to karva test

[MatthewMckee4]

Summary

• Add --dry-run flag to karva test that prints discovered tests without executing them
• Extract collect_tests() from run_parallel_tests() in karva_runner for reuse
• Output sorted test names as <test> module::function with a summary count (N tests collected)

Closes #465

Test plan

• test_dry_run — multiple files, verifies sorted output and count
• test_dry_run_empty — empty project, verifies 0 tests collected
• test_dry_run_with_path_filter — verifies --dry-run respects path arguments
• Full test suite passes (just test — 631 tests)
• Pre-commit checks pass (uvx prek run -a)

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing add-dry-run-flag (ba83f89) with main (8401165)}