Move Ctrl+C handler to karva_runner with config flag

[MatthewMckee4]

Summary

• Add create_ctrlc_handler field to ParallelTestConfig to control whether the shutdown signal handler is installed
• Move handler setup from karva crate into karva_runner, using OnceLock for idempotent initialization
• Remove the shutdown_rx: Option<&Receiver<()>> parameter from run_parallel_tests

This eliminates the panic when calling run_parallel_tests multiple times (e.g., in benchmarks) and cleans up the API.

Fixes #393

Test plan

• All existing tests pass
• New unit test test_shutdown_receiver_is_idempotent verifies idempotent initialization
• Manual test: Run cargo run test <path> and press Ctrl+C to verify graceful shutdown

🤖 Generated with Claude Code

[github-actions]

Project Diff Results

View diff

## pydantic-settings

1c1
< test result: FAILED. 376 passed; 33 failed; 4 skipped
---
> test result: FAILED. 377 passed; 32 failed; 4 skipped

## pydantic

1c1
< test result: FAILED. 8670 passed; 1451 failed; 265 skipped
---
> test result: FAILED. 8686 passed; 1435 failed; 265 skipped

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing issue-393-ctrlc-handler-config (403bee3) with main (b383518)}

⚠️ 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.

Summary

✅ 5 untouched benchmarks