perf(sdk): release the chat chunk buffer once the response is committed

On a successful turn the buffered chunks are dead weight (the recovered-partial
fallback prefers the onFinish message), yet they were retained through the
end-of-turn idle wait. Clear the buffer once the response is committed so a
completed turn no longer holds a full copy of its streamed output while idle.

Author: matt-aitken

packages/trigger-sdk/src/v3/ai.ts

@@ -7408,6 +7408,7 @@ function chatAgent<
                   if (capturedResponseMessage) {
                     responseCommitted = true;
                     capturedPartialResponse = capturedResponseMessage;
+                    turnBufferedChunks.length = 0;
                   }
 
                   if (runSignal.aborted) return "exit";