fix: preserve complete MCP call tool results

[eyesofish]

Summary

• preserve the complete successful MCP CallToolResult, including ordered content, structured content, metadata, and isError
• retain the legacy text_output field for backward compatibility
• cover mixed text/image and non-text-only results

Testing

• ./mvnw fmt:format
• ./mvnw -pl core -Dtest=AbstractMcpToolTest,ClaudeTest test
• ./mvnw -pl core test (1,751 tests, 0 failures)

Closes #1443

[google-cla]

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[hemasekhar-p]

Hi @eyesofish, thank you for your contribution! We appreciate you taking the time to submit this pull request. Since this is your first contribution, could you please complete the Google CLA so that we can proceed with the review?