Fix flaky test_reading_from_pipes on Python 3.13 with pytest-xdist

[Dev-iL]

Summary

This PR fixes the flaky test_reading_from_pipes test on Python 3.13 with pytest-xdist through two complementary fixes.

Fix 1: Defensive improvements

These changes address the symptom pathway where inherited mocks in forked processes could trigger warnings:

1. context.py: Changed hasattr() to getattr() to avoid triggering coroutine creation when checking for async methods on AsyncMock objects
2. test_supervisor.py: Added use_real_secrets_backends fixture to ensure real backend instances are used in subprocess tests, preventing inheritance of mocked backends across fork boundaries
3. supervisor.py: Fixed resource cleanup in _remote_logging_conn() with explicit del statements to prevent leaking client references
4. client.py: Fixed lru_cache binding by calling via type(self) instead of self to avoid carrying stale cache state across forked processes

Fix 2: Root cause fix

The actual source of the unawaited coroutine was in test_runtime_error_triggers_greenback_fallback in test_secrets.py:

1. The test mocked greenback.await_ to return a value directly without actually awaiting anything
2. When backend.get_connection() executed greenback.await_(self.aget_connection(conn_id)), a real coroutine was created
3. The mocked greenback.await_ returned immediately without awaiting that coroutine
4. When the orphaned coroutine was garbage collected, Python 3.13 raised RuntimeWarning: coroutine 'ExecutionAPISecretsBackend.aget_connection' was never awaited
5. When tests ran in parallel with pytest-xdist, this warning appeared in captured_logs of other tests, causing assertion failures

The fix: Modified the test to mock greenback.await_ with a side_effect that actually awaits the coroutine, and mock aget_connection to return the expected connection directly.

Both fixes together make the codebase more robust against async-related test flakiness on Python 3.13.

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[jscheffl]

Close and re-open to force a new CI run...

[potiuk]

Hmm. Mypy failed

[Dev-iL]

I did some more local testing, There was another flaky test in there. Let's hope it's fine now.

Please note that the AI introduced additional changes since your approval.

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

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Status	Branch	Result
❌	v3-1-test

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