#5293 fix(fastmcp): respect is_error tool results

Summary

• handle FastMCP tool results with is_error=True in direct_call_tool()
• map the error result through tool_error_behavior so non-retryable tool errors do not silently bypass policy
• add focused regression tests for both model_retry and error behaviors

Closes #5217

Testing

• uv run pytest tests/test_fastmcp.py -k "is_error_result or error_behavior"