An agent's tts_voice_id (e.g. "en-US-GuyNeural" for Edge) was applied
regardless of the selected TTS provider, causing OpenAI and other
providers to reject unknown voice IDs.
- Add internal/audio/voice_compat.go with IsVoiceCompatible,
GetProviderDefaultVoice, and FilterVoiceForProvider helpers
- Update TtsTool.resolveVoiceAndModel to report whether the voice
came from an agent override (voiceFromAgent)
- Apply applyVoiceCompat per-provider branch in Execute so
incompatible agent voices fall back to the provider's default
with a slog.Warn for operator visibility
- Add voice_compat_test.go with table-driven tests for all cases
Co-authored-by: Bruno Clermont <bruno.clermont@gmail.com>