Surface Pydantic model_dump failures explicitly

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Summary

serialize_pydantic calls model_dump and assumes it succeeds; exceptions should surface clearly as codec errors.

Failure mode

• A model with a failing model_dump raises an exception that bubbles up as a generic protocol error.

Repro (adversarial)

• Add a fixture Pydantic model whose model_dump raises (e.g., custom model_dump override).
• Call through NodeBridge and verify explicit error message.

Acceptance criteria

• model_dump errors surface as clear BridgeExecutionError messages.
• Adversarial tests cover a failing model_dump path.