fix: close cross-validation executor pools

[mameikagou]

Fixes #2355.

This applies the executor cleanup proposed in #2738 and adds focused regression coverage.

What changed

• Shut down internally created thread and process executors after cross-validation, including error paths.
• Preserve caller-owned executors and Dask clients.
• Cover repeated runs, mapping failures, and caller-owned executors.

Verification

• Focused diagnostics tests: 5 passed
• Python compile check
• git diff --check
• Pyrefly 0.50.0: 0 errors (18 existing suppressions), all 9 configured modules checked. The local hidden-directory exclusion heuristic was disabled; project includes/excludes were unchanged.

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[tcuongd]

LGTM but with a couple of suggestions. Type checking is also failing, PTAL. Thank you!

[tcuongd]

At this stage it probably makes more sense to check whether pool is an instance of concurrent.futures.Executor rather than manually assign _owns_pool.

[tcuongd]

Maybe it's easier to just subclass concurrent.futures.ThreadPoolExecutor?

[mameikagou]

Thanks, updated in 2cd2510. Cleanup now uses the Executor type guard while remaining limited to internally created pools. The tests subclass ThreadPoolExecutor and confirm that a caller-owned executor remains usable after cross-validation. All five focused tests and the configured Pyrefly 0.50.0 check pass locally.