fix: validate OpenAI TTS voice names, fall back to nova for unknown IDs

When the BatchTTSFallback chain passes an ElevenLabs voice ID to the
OpenAI provider, OpenAI returns 400 because it's not a valid voice
name. Now validates against the known set and falls back to 'nova'.

Author: jddunn

src/voice-pipeline/providers/OpenAIBatchTTS.ts

@@ -46,8 +46,16 @@ export class OpenAIBatchTTS implements IBatchTTS {
    * @param config - Optional voice, format, and speed overrides.
    * @returns The synthesized audio buffer with metadata.
    */
+  /** Valid OpenAI TTS voice names. */
+  private static readonly VALID_VOICES = new Set([
+    'nova', 'shimmer', 'echo', 'onyx', 'fable', 'alloy', 'ash', 'sage', 'coral',
+  ]);
+
   async synthesize(text: string, config?: BatchTTSConfig): Promise<BatchTTSResult> {
-    const voice = config?.voice ?? 'nova';
+    // Validate voice name — if an external voice ID (e.g. ElevenLabs) is passed
+    // via the fallback chain, fall back to 'nova' instead of sending it to OpenAI.
+    const rawVoice = config?.voice ?? 'nova';
+    const voice = OpenAIBatchTTS.VALID_VOICES.has(rawVoice) ? rawVoice : 'nova';
     const format = config?.format ?? 'mp3';
 
     const body: Record<string, unknown> = {