feat(translator): preserve input_audio fields in OpenAI request conversions

- Updated `ConvertOpenAIRequestToGemini` and `ConvertOpenAIRequestToCodex` to handle `input_audio`, retaining `data` and `format` fields.
- Added helper `openAIInputAudioMimeType` for determining MIME types from audio formats.
- Introduced unit tests to validate correct preservation of `input_audio` data and format.

Closes: #3447

internal/translator/codex/openai/chat-completions/codex_openai_request.go

@@ -193,6 +193,20 @@ func ConvertOpenAIRequestToCodex(modelName string, inputRawJSON []byte, stream b
 									msg, _ = sjson.SetRawBytes(msg, "content.-1", part)
 								}
 							}
+						case "input_audio":
+							if role == "user" {
+								audioData := it.Get("input_audio.data").String()
+								audioFormat := it.Get("input_audio.format").String()
+								if audioData != "" {
+									part := []byte(`{}`)
+									part, _ = sjson.SetBytes(part, "type", "input_audio")
+									part, _ = sjson.SetBytes(part, "data", audioData)
+									if audioFormat != "" {
+										part, _ = sjson.SetBytes(part, "format", audioFormat)
+									}
+									msg, _ = sjson.SetRawBytes(msg, "content.-1", part)
+								}
+							}
 						}
 					}
 				}

internal/translator/codex/openai/chat-completions/codex_openai_request_test.go

@@ -352,6 +352,39 @@ func TestToolCallOutputWithNonStringJSONContent(t *testing.T) {
 	}
 }
 
+func TestConvertOpenAIRequestToCodexPreservesInputAudio(t *testing.T) {
+	input := []byte(`{
+		"model": "gpt-5.5",
+		"messages": [
+			{
+				"role": "user",
+				"content": [
+					{"type": "text", "text": "Transcribe this audio verbatim."},
+					{"type": "input_audio", "input_audio": {"data": "SUQzBA==", "format": "mp3"}}
+				]
+			}
+		]
+	}`)
+
+	out := ConvertOpenAIRequestToCodex("gpt-5.5", input, true)
+	parts := gjson.GetBytes(out, "input.0.content").Array()
+	if len(parts) != 2 {
+		t.Fatalf("expected 2 content parts, got %d: %s", len(parts), gjson.GetBytes(out, "input.0.content").Raw)
+	}
+	if parts[0].Get("type").String() != "input_text" || parts[0].Get("text").String() != "Transcribe this audio verbatim." {
+		t.Fatalf("part 0: expected input_text with prompt text, got %s", parts[0].Raw)
+	}
+	if parts[1].Get("type").String() != "input_audio" {
+		t.Fatalf("part 1: expected input_audio, got %s", parts[1].Raw)
+	}
+	if parts[1].Get("data").String() != "SUQzBA==" {
+		t.Fatalf("part 1: expected audio data to be preserved, got %s", parts[1].Get("data").String())
+	}
+	if parts[1].Get("format").String() != "mp3" {
+		t.Fatalf("part 1: expected audio format mp3, got %s", parts[1].Get("format").String())
+	}
+}
+
 // Parallel tool calls: assistant invokes 3 tools at once, all call_ids
 // and outputs must be translated and paired correctly.
 func TestMultipleToolCalls(t *testing.T) {

internal/translator/gemini/openai/chat-completions/gemini_openai_request.go

@@ -210,6 +210,14 @@ func ConvertOpenAIRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 							} else {
 								log.Warnf("Unknown file name extension '%s' in user message, skip", ext)
 							}
+						case "input_audio":
+							audioData := item.Get("input_audio.data").String()
+							if audioData != "" {
+								mimeType := openAIInputAudioMimeType(item.Get("input_audio.format").String())
+								node, _ = sjson.SetBytes(node, "parts."+itoa(p)+".inlineData.mime_type", mimeType)
+								node, _ = sjson.SetBytes(node, "parts."+itoa(p)+".inlineData.data", audioData)
+								p++
+							}
 						}
 					}
 				}
@@ -438,3 +446,26 @@ func openAIToolCallGeminiThoughtSignature(toolCall gjson.Result) string {
 
 // itoa converts int to string without strconv import for few usages.
 func itoa(i int) string { return fmt.Sprintf("%d", i) }
+
+func openAIInputAudioMimeType(audioFormat string) string {
+	switch audioFormat {
+	case "", "wav":
+		return "audio/wav"
+	case "mp3":
+		return "audio/mpeg"
+	case "ogg":
+		return "audio/ogg"
+	case "flac":
+		return "audio/flac"
+	case "aac":
+		return "audio/aac"
+	case "webm":
+		return "audio/webm"
+	case "pcm16":
+		return "audio/pcm"
+	case "g711_ulaw", "g711_alaw":
+		return "audio/basic"
+	default:
+		return "audio/" + audioFormat
+	}
+}

internal/translator/gemini/openai/chat-completions/gemini_openai_request_test.go

@@ -26,3 +26,35 @@ func TestConvertOpenAIRequestToGemini_StripsTrailingAssistantPrefill(t *testing.
 		t.Fatalf("final remaining role = %q, want %q", got, "user")
 	}
 }
+
+func TestConvertOpenAIRequestToGeminiPreservesInputAudio(t *testing.T) {
+	inputJSON := `{
+		"model": "gpt-5.5",
+		"messages": [
+			{
+				"role": "user",
+				"content": [
+					{"type": "text", "text": "Transcribe this audio verbatim."},
+					{"type": "input_audio", "input_audio": {"data": "SUQzBA==", "format": "mp3"}}
+				]
+			}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToGemini("gemini-3.1-pro-high", []byte(inputJSON), false)
+	resultJSON := gjson.ParseBytes(result)
+	parts := resultJSON.Get("contents.0.parts").Array()
+
+	if len(parts) != 2 {
+		t.Fatalf("parts length = %d, want 2. parts=%s", len(parts), resultJSON.Get("contents.0.parts").Raw)
+	}
+	if got := parts[0].Get("text").String(); got != "Transcribe this audio verbatim." {
+		t.Fatalf("text part = %q, want prompt text", got)
+	}
+	if got := parts[1].Get("inlineData.mime_type").String(); got != "audio/mpeg" {
+		t.Fatalf("audio mime_type = %q, want %q", got, "audio/mpeg")
+	}
+	if got := parts[1].Get("inlineData.data").String(); got != "SUQzBA==" {
+		t.Fatalf("audio data = %q, want %q", got, "SUQzBA==")
+	}
+}