fix: prevent responseListeners memory leak in CoreMessenger (#11901)

Response listeners were only removed when data contained `"done": true`,
but most message types (file events, autocomplete, nextEdit, etc.)
return responses without a `done` field. This caused every fire-and-forget
request to leak its callback lambda permanently in the map.

Over a long session with heavy autocomplete and file navigation, this
could accumulate hundreds of thousands of entries (40-100+ MB).

Fix: invert the removal logic — remove the listener by default, and
only keep it when `done == false` (i.e., a streaming response is still
in progress). This preserves existing streaming behavior while ensuring
all non-streaming responses are cleaned up immediately.