fix(sidecars): finish timeout cleanup before retrying

[debpalash]

A sidecar timeout could return after stdout closed but before its owning process exited. A fast retry could reuse that dying process; an already-running watchdog could also target a replacement through mutable state.

The watchdog now captures its original child, kills and waits for that child with a bounded reap, and receive cleanup joins the watchdog before allowing recovery. The implementation uses the same process APIs on all supported platforms.

Validation: two deterministic regressions failed before the fix; 109 focused cross-engine tests and all 336 backend tests pass with offline, empty Hugging Face caches. Documentation updated.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 53ff367 and af7f1ce.

📒 Files selected for processing (5)

• CHANGELOG.md
• backend/services/subprocess_backend.py
• backend/tests/test_omnivoice_subprocess.py
• docs/engines/omnivoice.md
• tests/test_resolve_heartbeat_1414.py

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[greptile-apps]

Greptile Summary

The PR makes subprocess timeout recovery wait for its captured watchdog, quarantines children that cannot be reaped within the bounded wait, and prevents stale watchdogs from targeting replacement processes.

• Captures the original child when starting the timeout watchdog.
• Blocks spawning or reusing a sidecar while timed-out process cleanup remains incomplete.
• Adds regression coverage for cleanup ordering, replacement safety, and failed bounded reaping.

Important Files Changed

Filename	Overview
backend/services/subprocess_backend.py	Adds captured-child timeout handling, watchdog joining, and quarantined cleanup retries.
backend/tests/test_omnivoice_subprocess.py	Adds deterministic coverage for timeout reaping, replacement isolation, and cleanup quarantine behavior.
tests/test_resolve_heartbeat_1414.py	Uses normal backend initialization so the existing resolution-heartbeat test includes required cleanup state.

_{Reviews (3): Last reviewed commit: "test: construct heartbeat backend with l..." | Re-trigger Greptile}