Fix pyright error in test_elicitation.py

Add None check for requestedSchema before accessing properties to fix
type narrowing error that was blocking CI.

Author: cbcoutinho

tests/server/fastmcp/test_elicitation.py

@@ -242,6 +242,7 @@ async def defaults_tool(ctx: Context[ServerSession, None]) -> str:
     async def callback_schema_verify(context: RequestContext[ClientSession, None], params: ElicitRequestParams):
         # Verify the schema includes defaults
         schema = params.requestedSchema
+        assert schema is not None, "Schema should not be None for form mode elicitation"
         props = schema["properties"]
 
         assert props["name"]["default"] == "Guest"