Fix MCPServer.call_tool return type

[FU-max-boop]

Motivation and Context

Closes #1251.

MCPServer.call_tool always requests converted tool results from the tool manager, so its return annotation should match the actual converted shapes: content-only sequences, structured (content, structured_content) tuples, or direct CallToolResult values. The previous dict[str, Any] return type also left an unreachable handler branch in _handle_call_tool.

How Has This Been Tested?

• python3 -m py_compile src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
• git diff --check
• uv run --frozen --no-default-groups --with pytest --with inline-snapshot pytest -c /dev/null tests/server/mcpserver/test_server.py -k 'call_tool_direct_returns_structured_tuple or tool_structured_output_primitive'

Breaking Changes

None. This is a type annotation and cleanup change with direct coverage for the runtime return shape.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

I kept the scope limited to the incorrect direct call_tool type and the unreachable code path it created. I attempted the full project-style uv dev-tool run locally, but dependency downloads for dev tooling stalled; the targeted regression test above passed.