Reject dicts with non-string keys explicitly

[bbopen]

Summary

Python dicts with non-string keys can fail JSON serialization; this should be detected and surfaced explicitly.

Failure mode

• json.dumps raises TypeError for non-string keys.
• Failure surfaces as a protocol error without actionable messaging.

Repro (adversarial)

• Add a fixture that returns {1: "one", ("a", "b"): "tuple"}.
• Call through NodeBridge and verify explicit error message.

Acceptance criteria

• Non-string dict keys are rejected with a clear error before encoding.
• Adversarial tests cover this case.