[Test] Add unit tests for audio stage input processors in PD disaggregation

Add 59 CPU-only tests covering the thinker->talker and talker->code2wav
transition functions that are critical for audio generation in PD mode.
Tests verify embedding merge correctness, TTS embed fallback, codec code
flattening, chunked processing, and full PD audio pipeline integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ahengljh