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/models/capabilities.ts

@@ -31,7 +31,9 @@ const AUDIO_INCAPABLE_MODELS = new Set<string>([
  * Returns true if the model can receive raw audio data directly via its API,
  * false if audio must be transcribed to text before sending.
  */
-export function supportsAudioInput(provider: string, model: string): boolean {
+export function supportsAudioInput(provider: string, model: string, override?: boolean): boolean {
+  if (override !== undefined) return override;
+
   // Provider must be in the capable set
   if (!AUDIO_CAPABLE_PROVIDERS.has(provider)) {
     return false;