fix(audio-cpp): forward voice reference transcripts

Saved voices send ref_text, but Fish Audio requires reference_text.
Derive the canonical parameter while preserving explicit overrides.
Both TTS modes use the shared builder.

Add regression cases and document the parameter alias.

Assisted-by: Codex:gpt-6

backend/cpp/audio-cpp/generation_request.cpp

@@ -135,6 +135,13 @@ build_tts_request(const backend::TTSRequest &request,
         task.options["language"] = request.language();
     }
 
+    // Saved voice profiles send ref_text; Fish Audio reads reference_text.
+    // Derive the alias before copying params so an explicit canonical key wins.
+    const auto reference_text = request.params().find("ref_text");
+    if (reference_text != request.params().end()) {
+        task.options["reference_text"] = reference_text->second;
+    }
+
     // LAST, so an explicit params entry wins over anything derived above. That
     // matters for "caption": a caller who sets params[caption] has named the
     // exact string they want, and it must not be overwritten by `instructions`.