fix: subprocess isolate to close timeout-immune tree-sitter hang

[seroperson]

Closes #1289

Summary

• Run the per-PR parse + cursor walk in a spawn-context multiprocessing.Pool(processes=1) with a hard SCORING_SUBPROCESS_BUDGET_S = 5.0s wall-clock. On timeout or worker crash the worker is killed and the PR is zero-scored with scoring_method='skipped-isolation-timeout'; the round continues.
• Register an atexit hook (isolated_scoring.shutdown) so the multiprocessing resource_tracker does not log spurious leaked-semaphore warnings.

Follow-up to #1276.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

Existing tests cover the subprocess isolation, but I've added several more to ensure timeout hanging works correctly.

[plind-junior]

@seroperson In which case does this edge case happen? I assume its only when the PR file size is over 1MB. wdyt?

[seroperson]

@seroperson In which case does this edge case happen? I assume its only when the PR file size is over 1MB. wdyt?

It's not exactly about an edge case, but a specifically structured file which makes parser hang regardless of defined timeout. The size doesn't matter. To not disclosure details publicly, they were already sent in DM to the maintainers.

[anderdc]

can you set SCORING_SUBPROCESS_BUDGET_S = 5.0 to match TREE_SITTER_PARSE_TIMEOUT_MICROS = 2_000_000 timing of 2 seconds

[anderdc]

Sorry, my earlier ask was off — I read the budget as per-file, but SCORING_SUBPROCESS_BUDGET_S bounds the whole PR (the for file in file_changes loop plus pickling in/out), while the 2s TREE_SITTER_PARSE_TIMEOUT_MICROS is per-file. At 2.0s a multi-file PR, or one slow-but-legit file plus IPC, can blow the budget and zero-score the whole PR. Please set it back to 5s, or whatever value you think gives enough headroom over the 2s per-file parse timeout. Worth noting it should also cover worker cold-start: right after a timeout resets the pool, the next PR pays a fresh spawn + bittensor/tree-sitter import inside the same window.