fix(google): restore session resumption opt-in on first connect

Always send sessionResumption: {} to opt in on first connect.
The previous fix omitted it entirely when no handle existed, creating
a deadlock where the server never enabled resumption tracking.

Author: swayamg20

.changeset/fix-gemini-31-live-api.md

@@ -10,4 +10,4 @@ Two paths were broken with Gemini 3.1:
 
 2. **`generateReply()`** — was using `sendClientContent` to trigger generation; Gemini 3.1 rejects this with "Request contains an invalid argument." Now uses `sendRealtimeInput({ text })` instead, which works across all Live API models.
 
-Also fixes spurious empty `tools` and `sessionResumption` fields being sent in the session setup config when they have no values, which could cause rejections on strict model versions.
+Also fixes spurious empty `tools` field being sent when there are no declarations. Restores session resumption opt-in: always sends `sessionResumption: {}` on first connect (so the server enables tracking and sends `sessionResumptionUpdate` events), and includes the handle on subsequent connects.

plugins/google/src/beta/realtime/realtime_api.ts

@@ -1254,7 +1254,7 @@ export class RealtimeSession extends llm.RealtimeSession {
       outputAudioTranscription: opts.outputAudioTranscription,
       sessionResumption: this.sessionResumptionHandle
         ? { handle: this.sessionResumptionHandle }
-        : undefined,
+        : {}, // empty object opts in to resumption on first connect
     };
 
     // Add generation fields at TOP LEVEL (NO generationConfig!)