feat(tts): add Fish Audio as a text-to-speech provider

Registry entry plus one config-driven FORMAT_HANDLERS handler. The model id
travels in an HTTP `model` header rather than the JSON body, and the voice is
a reference_id (preset or cloned voice model).

Closes #2411

open-sse/providers/registry/index.js

@@ -119,6 +119,7 @@ import p116 from "./tokenrouter.js";
 import p117 from "./selfhosted-stt.js";
 import p118 from "./selfhosted-tts.js";
 import p119 from "./selfhosted-embedding.js";
+import p120 from "./fish-audio.js";
 
 export default [
   p0,
@@ -239,4 +240,5 @@ export default [
   p117,
   p118,
   p119,
+  p120,
 ];