fix(types): correct FastMCP.call_tool and convert_result return types (#1251)

[SAY-5]

Summary

Fixes #1251 (labelled ready for work). FastMCP.call_tool was annotated as returning Sequence[ContentBlock] | dict[str, Any], but it always passes convert_result=True to the tool manager and the actual return shape from FuncMetadata.convert_result is one of:

• Sequence[ContentBlock] — tool with no output schema returning a regular value
• CallToolResult — tool that returned a CallToolResult directly
• tuple[Sequence[ContentBlock], dict[str, Any]] — tool with an output schema (the second element is the structured content; not a bare dict)

So the existing annotation was wrong on two counts: it claimed a bare dict instead of the tuple shape, and it omitted the CallToolResult branch.

Changes

• src/mcp/server/mcpserver/server.py:402 — update FastMCP.call_tool return type to the precise three-arm union and add a docstring listing each branch.
• src/mcp/server/mcpserver/utilities/func_metadata.py:91 — replace Any on convert_result with the same three-arm union and tighten the docstring; reuses the existing Sequence, ContentBlock, and CallToolResult imports already in scope.

No runtime behaviour change — pure typing fix.

Test plan

• pytest tests/server/mcpserver/test_server.py tests/server/mcpserver/test_func_metadata.py tests/server/mcpserver/test_tool_manager.py — 172 passed
• inspect.signature round-trip on both functions returns the new annotations as expected