[Bug]: Stream TTS via proxy by deferring OpenAI streaming context; add verify script

[mfittko]

Fixes BerriAI#14891

Upstream issue: BerriAI#14891

Summary

• Problem: /v1/audio/speech (OpenAI TTS) via the proxy buffered or yielded zero bytes, preventing progressive playback.
• Root cause: OpenAI TTS adapter returned an object tied to a closed streaming context; proxy generator iterated the stream incorrectly.
• Fix: defer opening the OpenAI streaming context until aiter_bytes() consumption and iterate the async iterator correctly. Keep sync speech() unchanged. Add a small script to verify streaming behavior.

Changes

• OpenAI provider (async TTS): return a deferred streaming object whose aiter_bytes() opens with_streaming_response.create(...) and yields chunks.
• Proxy generator: ensure we iterate bytes correctly from the returned stream.
• Scripts: add scripts/verify_tts_streaming.py to validate headers, time-to-first-byte, and bytes received.
• Tests: add tests/litellm/test_tts_deferred_streaming.py to minimally validate deferred streaming yields bytes (skipped if async plugin missing).

Checklist

• I have added at least one test under tests/litellm/
• I have attached/provided local test results
• My PR scope is isolated and solves one problem (TTS streaming)

Notes

• For browser clients, use response.body streaming (ReadableStream) instead of arrayBuffer()/blob() for progressive TTS playback.