Add integration test: NATSListener rejects malformed real NATS subject

[mvillmow]

The current tests for NATSListener._on_task_event are pure unit tests using mocked TaskClaimer. There is no integration test that wires up a real (or simulated) NATS message delivery and verifies that a message with a malformed subject is dropped end-to-end. An integration test in tests/integration/ would confirm the validation holds through the full dispatch chain.

Follow-up from #137

[mvillmow]

[myrmidon-swarm classification: HARD — deferred]

This issue has been classified as HARD by the myrmidon swarm triage pass and is deferred from the current implementation run.

Reasons for HARD classification typically include:

• End-to-end NATS/JetStream integration requiring a live NATS server
• TSan-incompatible dependencies (e.g. concurrentqueue + -fsanitize=thread)
• Architectural cross-subsystem wiring (TransparentBridge, MessageBus ↔ NATSListener)
• Multi-phase changes requiring other HARD issues to land first
• Real external service integration tests

This issue will be addressed in a future implementation wave.