Fix diagnostics cancellation interrupt race

[renkun-ken]

Summary

• retire pooled callr workers when a running task is cancelled
• prevent late interrupts from leaking into replacement diagnostics
• add regression coverage for persistent-session cancellation

Verification

• task manager tests, including real subprocess tests
• lintr diagnostics tests
• 100-iteration cancellation stress test with zero errors or stalls
• formatting and LSP 3.18 tests with a writable cache