fix: preserve FastMCP tool error results

[he-yufeng]

Summary

• skip structured-output validation for CallToolResult(isError=True) in FastMCP's v1.x result conversion path
• keep validation unchanged for successful CallToolResult values with annotated output schemas
• add a regression test for annotated tool error results without structuredContent

Fixes #2429.

To verify

• python -m pytest tests\server\fastmcp\test_func_metadata.py -q -k "tool_call_result"
• python -m pytest tests\server\fastmcp\test_func_metadata.py -q
• python -m py_compile src\mcp\server\fastmcp\utilities\func_metadata.py tests\server\fastmcp\test_func_metadata.py
• python -m ruff check src\mcp\server\fastmcp\utilities\func_metadata.py tests\server\fastmcp\test_func_metadata.py
• git diff --check

[he-yufeng]

Updated the test assertion to narrow the tool result content before accessing text, which fixes the pyright failure from the first CI run.

Additional local verification:

• uv run pyright tests/server/fastmcp/test_func_metadata.py
• uv run python -m pytest tests/server/fastmcp/test_func_metadata.py -q
• .venv\Scripts\python.exe -m ruff check src/mcp/server/fastmcp/utilities/func_metadata.py tests/server/fastmcp/test_func_metadata.py