test: Enhance test coverage for SyncMcpToolCallback edge cases

[alxkm]

Add additional test coverage for edge cases in SyncMcpToolCallback.

Changes

• Empty Response Handling: Test that empty content lists return "[]" JSON array
• Multiple Content Items: Verify multiple TextContent items are properly serialized as JSON objects with text fields
• Non-Text Content Support: Ensure ImageContent is correctly serialized with data and mimeType fields

Coverage Improvements

• API Contract Documentation: Tests document exact JSON serialization format
• Regression Prevention: Will catch changes to response format in future
• Edge Case Coverage: Handles scenarios where tools return various content types