fix: use pydantic Field with alias for _meta fields

Pydantic treats fields starting with underscore as private/hidden. To fix this,
we need to use Field with alias='_meta' to properly handle these fields while
keeping the external API unchanged. This fixes #103 where meta fields were not
being properly assigned in request contexts.

src/mcp/shared/session.py

@@ -221,7 +221,7 @@ class BaseSession(
                     )
                     responder = RequestResponder(
                         request_id=message.root.id,
-                        request_meta=validated_request.root.params._meta
+                        request_meta=validated_request.root.params.meta
                         if validated_request.root.params
                         else None,
                         request=validated_request,