fix(test): make concurrent invocation tests deterministic

[strands-agent]

Description

Fixes flaky concurrent invocation tests that were failing intermittently on macOS Python 3.13.

Root Cause

The tests test_agent_concurrent_call_raises_exception and test_agent_concurrent_structured_output_raises_exception used timing-based synchronization (time.sleep(0.05)) to ensure the second thread started while the first thread held the invocation lock. On faster machines (particularly macOS Python 3.13), thread scheduling could allow the first invocation to complete before the second thread started, causing both threads to succeed instead of one raising ConcurrencyException.

Solution

Replace timing-based synchronization with proper thread coordination using threading.Event:

1. Extended SlowMockedModel to accept an optional signal_event parameter
2. The model signals when it starts streaming (at which point the lock is held)
3. Tests now use lock_acquired.wait(timeout=2.0) instead of time.sleep(0.05)
4. This ensures deterministic test behavior regardless of machine speed

Changes

• SlowMockedModel now accepts delay and signal_event parameters
• Tests wait for explicit signal rather than arbitrary sleep duration
• Renamed local lock variable to results_lock for clarity

Related Issues

This flaky test was blocking PR #1491 and potentially other PRs with spurious CI failures.

Type of Change

Bug fix (test flakiness)

Testing

• Ran the concurrent tests 5+ times locally, all passed consistently
• hatch fmt passes

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective or that my feature works
• My changes generate no new warnings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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