fix: add branch coverage pragmas to test helper

Github-Issue: #1283

Author: felixweinberger

tests/issues/test_1283_idle_session_cleanup.py

@@ -50,9 +50,9 @@ async def mock_send(message: Message) -> None:
 
 def _extract_session_id(sent_messages: list[Message]) -> str:
     for msg in sent_messages:
-        if msg["type"] == "http.response.start":
+        if msg["type"] == "http.response.start":  # pragma: no branch
             for name, value in msg.get("headers", []):
-                if name.decode().lower() == MCP_SESSION_ID_HEADER.lower():
+                if name.decode().lower() == MCP_SESSION_ID_HEADER.lower():  # pragma: no branch
                     return value.decode()
     raise AssertionError("Session ID not found in response headers")  # pragma: no cover