fix(server): allow tools with outputSchema to surface errors via unstructured content

[SAY-5]

Per #2429, a low-level tool with an outputSchema cannot return a tool-execution failure (e.g. "Resource not found") via unstructured TextContent because the call_tool decorator unconditionally validates the result against outputSchema and replaces the original error message with a hard "outputSchema defined but no structured output returned" error.

This PR mirrors the TypeScript SDK fix in modelcontextprotocol/typescript-sdk#655: when an outputSchema-bound tool returns no structured content, treat the unstructured payload as a tool-error result (isError=True) instead of replacing it. Tools that do return structured output keep their existing validation contract, and the explicit CallToolResult-returning path is unchanged.

Closes #2429

[SAY-5]

Pushed 5239245 updating test_content_only_with_output_schema_error (now renamed *_surfaces_error) to assert the new contract, isError=True plus the tool's original unstructured TextContent preserved verbatim. pytest tests/server/test_lowlevel_output_validation.py runs all 9 cases green locally.

[maxisbey]

Thanks for the PR, and sorry it sat here without a proper review. The 1.x line only takes critical and security fixes now, and this one is below that bar.

We're closing most of the open PR backlog. v2 is out and changed a lot of the SDK, so many older PRs no longer apply as written, and we're a small team that realistically doesn't have the capacity to work through the rest.

If this still matters to you on v2, the most useful thing you can do is open an issue (or comment on the existing one) with your use case and a repro. Hearing why it matters to you is what we use to decide what to prioritise.

_{AI Disclaimer}