fix(sse): iterate async TTS generator from sync context without async io.run; return proper SSE chunks

[DanTulovsky]

This is a fix for: #34

Full disclosure, fix provided by GPT5. It does seem to fix the error though. It said:

So the exact failure is: asyncio.run expects a coroutine object, but it was given an async generator (_generate_audio_stream(...)), producing “a coroutine was expected, got <async_generator ...>”, which your SSE wrapper then emits as a JSON error.

Status: Reviewed app/tts_handler.py and app/server.py and identified a type mismatch between an async generator and asyncio.run as the cause of the SSE error.

Cause: asyncio.run(_generate_audio_stream(...)) where _generate_audio_stream is an async generator.
Effect: SSE path emits {"type":"error","error":"a coroutine was expected, got <async_generator ...>"}.
The non-SSE path works because it runs a proper coroutine (_generate_audio) via asyncio.run.```

Before:

❯ curl -X POST http://localhost:5050/v1/audio/speech
-H "Content-Type: application/json"
-H "Authorization: Bearer your_api_key_here"
-d '{
"model": "tts-1",
"input": "This will stream as Server-Sent Events with JSON data containing base64-encoded audio chunks.",
"voice": "alloy",
"stream_format": "sse"
}'
data: {"type": "error", "error": "a coroutine was expected, got <async_generator object _generate_audio_stream at 0x2aaaae3a7450>"}

After:

❯ curl -X POST http://localhost:5050/v1/audio/speech
-H "Content-Type: application/json"
-H "Authorization: Bearer your_api_key_here"
-d '{
"model": "tts-1",
"input": "This will stream as Server-Sent Events with JSON data containing base64-encoded audio chunks.",
"voice": "alloy",
"stream_format": "sse"
}'
data: {"type": "speech.audio.delta", "audio": "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"}

data: {"type": "speech.audio.delta", "audio": "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"}