fix(security): circuit breaker for tirith crashes to prevent agent hangs (#41400)

[kshitijk4poor]

Summary

Tirith security scanner crashes (SIGSEGV, FileNotFoundError, timeout) no longer cause 20-minute agent hangs — after 3 consecutive failures, a circuit breaker disables tirith for the rest of the process.

Root cause (#41400)

When tirith_path points to a non-existent or broken binary, every tool invocation triggers a crash (exit code -11/SIGSEGV, FileNotFoundError, etc.). With fail_open: true, the tool proceeds anyway, and the agent retries the same tool call — creating a 20-minute hang where no responses are delivered. 2,704+ crashes were logged over 7 days, all hidden behind fail_open.

Changes

• tools/tirith_security.py: circuit breaker pattern — tracks consecutive failures across all three failure paths (OSError, TimeoutExpired, unexpected exit code including signals). After 3 consecutive failures (_CRASH_LIMIT), sets _circuit_open = True and short-circuits all subsequent calls with {"action": "allow", "summary": "tirith disabled (circuit breaker)"}. Successful execution resets the counter.
• tests/tools/test_tirith_security.py: existing tests updated to account for circuit breaker behavior after 3 failures.

Validation

	Before	After
SIGSEGV crash (fail_open=true)	crash every call, agent retries forever	3 crashes → circuit opens → no more spawns
FileNotFoundError	same hang pattern	circuit breaker opens after 3
Timeout	same hang pattern	circuit breaker opens after 3
Successful execution	n/a	resets crash counter

• scripts/run_tests.sh tests/tools/test_tirith_security.py — 95/95 passed.
• E2E: verified circuit breaker opens after 3 SIGSEGV/FileNotFoundError/Timeout failures, short-circuits subsequent calls, resets on success, and doesn't trigger when tirith is disabled.

Salvaged from #41446 by @kyssta-exe. Chosen over #41423 (@iamlukethedev) because it's simpler (66 lines vs 262), uses module-level globals without a lock (matching the existing _warn_once pattern in the same file), and doesn't add a separate test file with placeholder assertions. #41423 uses a thread lock and has a placeholder test (assert result is not None or result is None).

Closes #41400