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

Replaces #1446 (accidentally closed; GitHub blocks reopening after the head branch was force-pushed).

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