test: replace multiprocessing with threading for server test coverage

Replace multiprocessing.Process with threading.Thread + uvicorn.Server
for test server fixtures (basic_server, json_server, resumable_server)
so coverage.py can track server-side code in the same process.

Changes:
- Add _start_server_thread() helper using uvicorn.Server in a daemon thread
- Graceful shutdown via server.should_exit instead of proc.kill()
- Remove 8 pragma: no cover from test fixtures (no longer needed)
- Add 8 new tests covering previously-uncovered branches
- Remove dead run_server() function and unused http_client fixture
- Convert pragma: no cover to pragma: lax no cover in source files
  for non-deterministic coverage lines (thread timing dependent)
- Add pragma: no branch for partial branch coverage on guard lines

Author: perhapzz

src/mcp/server/session.py

@@ -200,7 +200,7 @@ async def _received_notification(self, notification: types.ClientNotification) -
             case types.InitializedNotification():
                 self._initialization_state = InitializationState.Initialized
             case _:
-                if self._initialization_state != InitializationState.Initialized:  # pragma: no cover
+                if self._initialization_state != InitializationState.Initialized:  # pragma: lax no cover
                     raise RuntimeError("Received notification before initialization was complete")
 
     async def send_log_message(
@@ -222,7 +222,7 @@ async def send_log_message(
             related_request_id,
         )
 
-    async def send_resource_updated(self, uri: str | AnyUrl) -> None:  # pragma: no cover
+    async def send_resource_updated(self, uri: str | AnyUrl) -> None:
         """Send a resource updated notification."""
         await self.send_notification(
             types.ResourceUpdatedNotification(
@@ -446,7 +446,7 @@ async def elicit_url(
             metadata=ServerMessageMetadata(related_request_id=related_request_id),
         )
 
-    async def send_ping(self) -> types.EmptyResult:  # pragma: no cover
+    async def send_ping(self) -> types.EmptyResult:  # pragma: lax no cover
         """Send a ping request."""
         return await self.send_request(
             types.PingRequest(),
@@ -478,11 +478,11 @@ async def send_resource_list_changed(self) -> None:
         """Send a resource list changed notification."""
         await self.send_notification(types.ResourceListChangedNotification())
 
-    async def send_tool_list_changed(self) -> None:  # pragma: no cover
+    async def send_tool_list_changed(self) -> None:  # pragma: lax no cover
         """Send a tool list changed notification."""
         await self.send_notification(types.ToolListChangedNotification())
 
-    async def send_prompt_list_changed(self) -> None:  # pragma: no cover
+    async def send_prompt_list_changed(self) -> None:  # pragma: lax no cover
         """Send a prompt list changed notification."""
         await self.send_notification(types.PromptListChangedNotification())