[Feature] Add regression tests for _classify_worker_death() and probe_capabilities() Windows platform guards

[Harshitmishra001]

Describe the bug

There is no existing test coverage for _classify_worker_death() in src/nooa/runtime/sandbox/executor.py or for the platform-guard behaviour of probe_capabilities() / apply_rlimits() in src/nooa/runtime/sandbox/guards.py.

Both functions were patched in #92 to guard against missing POSIX-only constants (signal.SIGXCPU, signal.SIGKILL) and the missing resource module on Windows. Without regression tests, any future refactor could silently revert those guards and the bugs would return undetected — CI would not catch it because the suite runs only on Linux today (see #95).

What's missing

• No test calls _classify_worker_death() with a simulated Windows environment (i.e. signal.SIGXCPU / signal.SIGKILL absent).
• No test calls probe_capabilities() with resource absent and asserts rlimit=False is returned instead of a crash.
• No test calls apply_rlimits() with resource absent and asserts it returns early rather than raising ModuleNotFoundError.

Confirmed by searching tests/ — zero hits for _classify_worker_death, SIGXCPU, SIGKILL, or WorkerDiedError in a death-classification context.

Expected behavior

Regression tests that:

1. Use monkeypatch.delattr to simulate a Windows signal module (remove SIGXCPU and SIGKILL) and assert _classify_worker_death() falls through cleanly without raising AttributeError.
2. Use monkeypatch to make import resource raise ImportError and assert probe_capabilities() returns Capabilities(..., rlimit=False) without crashing.
3. Same for apply_rlimits() — assert it returns early when resource is unavailable.

Environment

• Not platform-specific — these tests should run on Linux CI using monkeypatch to simulate the missing platform attributes.

Additional context

This is a follow-up to #92. The fixes and these regression tests will be submitted together in a single PR so the guards and their verification land atomically.

No code change is needed in the production files — only new test cases in tests/test_sandbox.py or a new tests/runtime/sandbox/test_guards_windows.py.