fix: cover experimental property cache-hit branch and fix completion test

Author: maxisbey

tests/experimental/tasks/server/test_server.py

@@ -299,7 +299,9 @@ async def test_update_capabilities_partial_handlers() -> None:
     """Test that update_capabilities skips list/cancel when only tasks/get is registered."""
     server = Server("test-partial")
     # Access .experimental to create the ExperimentalHandlers instance
-    _ = server.experimental
+    exp = server.experimental
+    # Second access returns the same cached instance
+    assert server.experimental is exp
 
     async def noop_get(ctx: ServerRequestContext, params: GetTaskRequestParams) -> GetTaskResult:
         raise NotImplementedError

tests/server/mcpserver/test_server.py

@@ -1412,10 +1412,9 @@ async def test_completion_decorator() -> None:
     @mcp.completion()
     async def handle_completion(
         ref: PromptReference, argument: CompletionArgument, context: CompletionContext | None
-    ) -> Completion | None:
-        if argument.name == "style":
-            return Completion(values=["bold", "italic", "underline"])
-        return None
+    ) -> Completion:
+        assert argument.name == "style"
+        return Completion(values=["bold", "italic", "underline"])
 
     async with Client(mcp) as client:
         ref = PromptReference(type="ref/prompt", name="test")