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/clients/openai.go

@@ -48,6 +48,11 @@ func (c OpenAIClient) Run(ctx context.Context, request Request) (Response, error
 	}
 	if endpointKind == "chat.completions" {
 		body = NormalizeChatCompletionRequestBody(body)
+		normalizedBody, normalizeErr := normalizeOpenAIChatAudioParts(body, request.Candidate)
+		if normalizeErr != nil {
+			return Response{}, normalizeErr
+		}
+		body = normalizedBody
 		applyOpenAIChatReasoningParams(body, request.Candidate)
 		body = FilterOpenAIChatRequestBody(body)
 	} else if request.Kind == "responses" {