MCPToolAdapter.invoke() and discover_tools() held the RLock during the
entire transport.call(), blocking all concurrent operations on the adapter.

The fix splits both methods into three phases:
1. Acquire lock — validate connection state and inputs
2. Release lock — make the transport call without holding the lock
3. Re-acquire lock (discover_tools only) — update shared state

Added Behave BDD tests verifying:
- Concurrent invocations complete in parallel (wall-clock < serial time)
- Concurrent discoveries complete in parallel
- Lock is provably not held during transport calls (cross-thread check)
- Validation still occurs under lock before transport call
- Disconnected adapter still raises RuntimeError under lock

ISSUES CLOSED: #10512

@@ -473,3 +491,4 @@
+        # Phase 1: Validate state and inputs under lock.
         with self._lock:
             if not self._connected:
                 msg = (