fix: remove SSE termination marker from DefaultStopSequences

The SSE stream termination marker string was incorrectly included in
DefaultStopSequences, causing Gemini to prematurely stop generating
output whenever the model produced text containing that marker.

The SSE-level protocol filtering in stream_transformer.go already
handles this marker correctly; it should not be a stop sequence for
the model's text generation.

backend/internal/pkg/antigravity/gemini_types.go

@@ -189,6 +189,5 @@ var DefaultStopSequences = []string{
 	"<|user|>",
 	"<|endoftext|>",
 	"<|end_of_turn|>",
-	"[DONE]",
 	"\n\nHuman:",
 }