fix: surface MCP structuredContent-only results

[Success6666]

Summary

MCP tool results that only return structuredContent now surface that payload through ToolResult.content when the normal content array produces no text output.

This keeps the existing metadata preservation from #8580, while making structured-only successful MCP responses visible to the standard tool-result -> model-message flow.

Fixes #8653

Type of change

• Bug fix

Details

• If MCP content already contains text, that text remains the primary tool output.
• If content is empty and structuredContent is present, the structured payload is serialized as JSON for ToolResult.content.
• metadata[structured_content] still preserves the original structured object.
• The structured-only regression test parses the serialized content as JSON before comparing the payload.

Tests

.venv\Scripts\python.exe -m pytest libs\agno\tests\unit\tools\test_mcp.py::test_mcp_tool_result_preserves_structured_content libs\agno\tests\unit\tools\test_mcp.py::test_mcp_tool_result_uses_structured_content_when_content_is_empty libs\agno\tests\unit\tools\test_mcp.py::test_mcp_tool_error_result_preserves_structured_content -q
.venv\Scripts\ruff.exe check libs\agno\agno\utils\mcp.py libs\agno\tests\unit\tools\test_mcp.py
.venv\Scripts\ruff.exe format --check libs\agno\agno\utils\mcp.py libs\agno\tests\unit\tools\test_mcp.py
git diff --check

[Copilot]

Pull request overview

This PR fixes how MCP tool results are surfaced through Agno’s standard tool-result → model-message pipeline when an MCP server returns a successful response with an empty content array but a populated structuredContent. In that structured-only case, the structured payload is now serialized to JSON and placed into ToolResult.content while still preserving the original object in ToolResult.metadata["structured_content"].

Changes:

• Serialize structuredContent into ToolResult.content when no text output is produced from the MCP content array.
• Add a helper (_structured_content_as_tool_output) to perform safe JSON serialization with fallbacks.
• Add/extend unit tests to cover both the text-content and structured-only success cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
libs/agno/agno/utils/mcp.py	Adds structuredContent-to-JSON fallback so structured-only MCP successes become visible in ToolResult.content.
libs/agno/tests/unit/tools/test_mcp.py	Adds a regression test ensuring structured-only results serialize into content and remain preserved in metadata.

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[Sebastiao-Assuncao]

Beautiful, thank you @Success6666