[Refactor] fish_speech: vendor DAC codec modules, drop external fish-speech dependency (#4871)

Signed-off-by: Yueqian Lin <linyueqian@outlook.com>

examples/offline_inference/text_to_speech/README.md

@@ -130,11 +130,7 @@ Text → [Stage 0: AR] → Speech Tokens → [Stage 1: DiT + HiFT] → Audio (24
 ## Fish Speech S2 Pro
 
 4B dual-AR text-to-speech model from FishAudio with the DAC codec at 44.1 kHz.
-
-### Prerequisites
-```bash
-pip install fish-speech
-```
+No extra packages are required; the DAC codec is vendored in vLLM-Omni.
 
 ### Quick start
 ```bash

examples/online_serving/text_to_speech/README.md

@@ -255,13 +255,8 @@ python examples/online_serving/text_to_speech/indextts2/speech_client.py \
 
 ## Fish Speech S2 Pro
 
-4B dual-AR TTS at 44.1 kHz. Server uses the DAC codec.
-
-### Prerequisites
-
-```bash
-pip install fish-speech
-```
+4B dual-AR TTS at 44.1 kHz. Server uses the DAC codec, which is vendored in
+vLLM-Omni (no extra packages required).
 
 ### Kvcache attention fast path