Add support for tool annotations via OpenAPI extensions

[parente]

Describe your changes

This PR implements the solution proposed in #252: specifying MCP tool annotations using an x-mcp-annotations OpenAPI extension via a route decorator.

Issue ticket number and link (if applicable)

#252

Screenshots of the feature / bugfix

@app.get(
        "/items/",
        response_model=List[Item],
        tags=["items"],
        operation_id="list_items",
        openapi_extra={
            "x-mcp-annotations": {
                "readOnlyHint": True,
                "openWorldHint": False,
            }
        },
    )
    async def list_items(...): ...

Checklist before requesting a review

• Added relevant tests
• Run ruff & mypy
• All tests pass