Remove over-conservative pragma: no cover annotations

The sleep(0) yield in the stateless request handler allows the event
loop to exercise exception paths that were previously unreachable with
mocked tests. These paths are legitimately reachable and the more
realistic test now covers them consistently:

- streamable_http_manager.py: except Exception in run_stateless_server
- streamable_http.py: except Exception in SSE pipeline (x2)
- streamable_http.py: if not self.mcp_session_id (always true in stateless mode)
- session.py: except Exception in receive loop cleanup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: wiggzz

src/mcp/server/streamable_http.py

@@ -626,14 +626,14 @@ async def sse_writer():  # pragma: lax no cover
                         # Then send the message to be processed by the server
                         session_message = self._create_session_message(message, request, request_id, protocol_version)
                         await writer.send(session_message)
-                except Exception:  # pragma: no cover
+                except Exception:
                     logger.exception("SSE response error")
                     await sse_stream_writer.aclose()
                     await self._clean_up_memory_streams(request_id)
                 finally:
                     await sse_stream_reader.aclose()
 
-        except Exception as err:  # pragma: no cover
+        except Exception as err:
             logger.exception("Error handling POST request")
             response = self._create_error_response(
                 f"Error handling POST request: {err}",
@@ -816,7 +816,7 @@ async def _validate_request_headers(self, request: Request, send: Send) -> bool:
 
     async def _validate_session(self, request: Request, send: Send) -> bool:
         """Validate the session ID in the request."""
-        if not self.mcp_session_id:  # pragma: no cover
+        if not self.mcp_session_id:
             # If we're not using session IDs, return True
             return True
 

src/mcp/server/streamable_http_manager.py

@@ -178,7 +178,7 @@ async def run_stateless_server(*, task_status: TaskStatus[None] = anyio.TASK_STA
                         self.app.create_initialization_options(),
                         stateless=True,
                     )
-                except Exception:  # pragma: no cover
+                except Exception:
                     logger.exception("Stateless session crashed")
 
         # Use a request-scoped task group instead of the global one.

src/mcp/shared/session.py

@@ -423,7 +423,7 @@ async def _receive_loop(self) -> None:
                     try:
                         await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error))
                         await stream.aclose()
-                    except Exception:  # pragma: no cover
+                    except Exception:
                         # Stream might already be closed
                         pass
                 self._response_streams.clear()