Refactor: Remove single-use TTS wrapper functions

[basnijholt]

Problem

The TTS synthesizer wrappers in agent_cli/services/tts.py are thin wrappers that only add **_kwargs to absorb unused config arguments:

async def _synthesize_speech_gemini(
    *,
    text: str,
    gemini_tts_cfg: config.GeminiTTS,
    logger: logging.Logger,
    **_kwargs: object,
) -> bytes | None:
    """Synthesize speech from text using Gemini TTS."""
    return await synthesize_speech_gemini(text=text, gemini_tts_cfg=gemini_tts_cfg, logger=logger)

This exists because the caller passes all TTS configs:

audio_data = await synthesizer(
    text=text,
    wyoming_tts_cfg=wyoming_tts_cfg,
    openai_tts_cfg=openai_tts_cfg,
    kokoro_tts_cfg=kokoro_tts_cfg,
    gemini_tts_cfg=gemini_tts_cfg,
    logger=logger,
)

Proposal

Add **_kwargs directly to the base functions in services/__init__.py and call them directly, removing the wrapper indirection.

Applies to:

• _synthesize_speech_gemini → synthesize_speech_gemini
• _synthesize_speech_openai → synthesize_speech_openai

Benefits

• Fewer indirections
• Follows "keep it DRY" and "simplest solution" principles