fix(audio): 兼容百炼 OpenAI 音频输入

为阿里云百炼 OpenAI-compatible Chat 统一保留标准 input_audio，并将旧版 audio_url 转换为 input_audio，同时根据 MIME 或扩展名补齐音频格式。

补充请求资产水合测试，确认 input_audio.data 会转换为裸 Base64 且保留 format。

验证：在 apps/api 执行 env -u AI_GATEWAY_TEST_DATABASE_URL go test ./... -count=1 全部通过。

apps/api/internal/runner/request_assets_test.go

@@ -45,6 +45,56 @@ func TestHydrateProviderRequestAssetsConvertsStrictBase64Field(t *testing.T) {
 	}
 }
 
+func TestHydrateProviderRequestAssetsConvertsInputAudioDataToRawBase64(t *testing.T) {
+	storageDir := t.TempDir()
+	fileName := "gateway-request-asset-audio.mp3"
+	payload := []byte("audio bytes")
+	if err := os.WriteFile(filepath.Join(storageDir, fileName), payload, 0o644); err != nil {
+		t.Fatalf("write request asset: %v", err)
+	}
+	service := &Service{cfg: config.Config{LocalUploadedStorageDir: storageDir}}
+	body := map[string]any{
+		"messages": []any{
+			map[string]any{
+				"role": "user",
+				"content": []any{
+					map[string]any{
+						"type": "input_audio",
+						"input_audio": map[string]any{
+							"data": map[string]any{
+								"assetRef": map[string]any{
+									"sha256":          "sha-input-audio",
+									"contentType":     "audio/mpeg",
+									"url":             "/static/uploaded/" + fileName,
+									"storageProvider": "local_static",
+								},
+								"url": "/static/uploaded/" + fileName,
+							},
+							"format": "mp3",
+						},
+					},
+				},
+			},
+		},
+	}
+
+	hydrated, err := service.hydrateProviderRequestAssets(context.Background(), body, store.RuntimeModelCandidate{})
+	if err != nil {
+		t.Fatalf("hydrate request assets: %v", err)
+	}
+	messages := hydrated["messages"].([]any)
+	message := messages[0].(map[string]any)
+	content := message["content"].([]any)
+	audioPart := content[0].(map[string]any)
+	inputAudio := audioPart["input_audio"].(map[string]any)
+	if got, want := stringFromAny(inputAudio["data"]), base64.StdEncoding.EncodeToString(payload); got != want {
+		t.Fatalf("unexpected hydrated input audio: got %q want %q", got, want)
+	}
+	if inputAudio["format"] != "mp3" {
+		t.Fatalf("input audio format was not preserved: %+v", inputAudio)
+	}
+}
+
 func TestHydrateProviderRequestAssetsConvertsBase64ArrayField(t *testing.T) {
 	storageDir := t.TempDir()
 	fileName := "gateway-request-asset-array.png"