feat(audio): add tests, token estimation, and config override for native audio

- Add capabilities.test.ts (18 tests) for supportsAudioInput()
- Add 15 audio tests to media.test.ts (hasAudio, stripAudioParts, attachmentToAudioSource)
- Add estimateAudioTokens() to tokens.ts (base64→bytes→duration→tokens)
- Update estimateMessageTokens() to include audio content parts
- Add 5 audio token tests to tokens.test.ts
- Add supports_audio config override to model schema
- Wire supports_audio from tier config through routing to capability check

Total tests: 1369 (was 1331, +38 audio-related)

src/config/schema.ts

@@ -52,6 +52,7 @@ const modelConfigBaseSchema = z.object({
   for: z.array(z.string()).optional(),
   num_gpu: z.number().optional(),
   context_window: z.number().optional(),
+  supports_audio: z.boolean().optional(),
 });
 
 const modelConfigSchema = modelConfigBaseSchema.extend({