Adds a Behave TDD issue-capture test for bug #10438: McpClient.start()
releases the threading.RLock after setting _state to STARTING but before
calling connect() and discover_tools(). Concurrent callers can both pass
the _started idempotency check and call discover_tools() multiple times.
The test uses @tdd_expected_fail so CI passes while the bug is unfixed.
A counting mock transport records connect() and discover_tools() calls.
Five threads call start() concurrently through a threading.Barrier to
maximise the chance of the race manifesting.
ISSUES CLOSED: #10402
HAL9000