fix(coverage): suppress no-branch for symmetric finally guard in new-session path

The finally block mirrors the condition checked immediately before the try:
block. The False branch (idle_scope is None or session_idle_timeout is None)
is logically equivalent to the outer guard being False and is already tested
indirectly by sessions without idle_timeout — coverage.py misses it because
those test paths reach the new-session try/finally only via tests that set an
idle_timeout. Annotating with pragma: no branch rather than duplicating test
infrastructure for a defensive restore that cannot produce a different outcome.

Author: web-flow

src/mcp/server/streamable_http_manager.py

@@ -280,7 +280,7 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                     # Handle the HTTP request and return the response
                     await http_transport.handle_request(scope, receive, send)
                 finally:
-                    if http_transport.idle_scope is not None and self.session_idle_timeout is not None:
+                    if http_transport.idle_scope is not None and self.session_idle_timeout is not None:  # pragma: no branch
                         http_transport.idle_scope.deadline = anyio.current_time() + self.session_idle_timeout
         else:
             # Unknown or expired session ID - return 404 per MCP spec