fix(voice-call): log TTS errors instead of swallowing silently

Fixes #2820

The `speak()` method returns `{ success: false, error }` instead of throwing
on failure. Two call sites were not checking this return value, causing TTS
errors to be silently swallowed:

1. `handleInboundResponse()` in webhook.ts - callers never received AI response audio
2. `continueCall()` in manager.ts - continued to "listening" state even when TTS failed

Changes:
- webhook.ts: Check `speak()` return value and log errors
- manager.ts: Check `speak()` return value in `continueCall()` and return early on failure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

extensions/voice-call/src/manager.ts

@@ -386,7 +386,10 @@ export class CallManager {
     }
 
     try {
-      await this.speak(callId, prompt);
+      const speakResult = await this.speak(callId, prompt);
+      if (!speakResult.success) {
+        return { success: false, error: speakResult.error || "Failed to speak prompt" };
+      }
 
       call.state = "listening";
       this.persistCallRecord(call);

extensions/voice-call/src/webhook.ts

@@ -352,7 +352,10 @@ export class VoiceCallWebhookServer {
 
       if (result.text) {
         console.log(`[voice-call] AI response: "${result.text}"`);
-        await this.manager.speak(callId, result.text);
+        const speakResult = await this.manager.speak(callId, result.text);
+        if (!speakResult.success) {
+          console.error(`[voice-call] Failed to speak: ${speakResult.error}`);
+        }
       }
     } catch (err) {
       console.error(`[voice-call] Auto-response error:`, err);