fix(apicompat): chat/completions file part 不再被静默丢弃，转换为 Responses input_file

/v1/chat/completions → Responses 转换层此前只处理 text 和 image_url 两种
content part，type:"file"（PDF 附件）被静默丢弃：请求返回 200、模型正常
回答，但 prompt 里没有文件（prompt_tokens 只剩纯文字）。

现将 file part 映射为 Responses API 的 input_file（filename/file_data/
file_id 透传），与同网关 /v1/responses + input_file 实测可用的格式一致。
无 file_data 且无 file_id 的空 file part 跳过，与空 image URL 行为一致。

参考上游 PR #2497（因混入无关改动未合并）。

backend/internal/pkg/apicompat/chatcompletions_responses_test.go

@@ -407,6 +407,79 @@ func TestChatCompletionsToResponses_WhitespaceOnlyBase64ImageURLSkipped(t *testi
 	assert.Equal(t, "Describe this", parts[0].Text)
 }
 
+func TestChatCompletionsToResponses_FilePartFileData(t *testing.T) {
+	content := `[{"type":"text","text":"Summarize the attached document"},{"type":"file","file":{"filename":"document.pdf","file_data":"data:application/pdf;base64,JVBERi0xLjQ="}}]`
+	req := &ChatCompletionsRequest{
+		Model: "gpt-4o",
+		Messages: []ChatMessage{
+			{Role: "user", Content: json.RawMessage(content)},
+		},
+	}
+	resp, err := ChatCompletionsToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 1)
+
+	var parts []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[0].Content, &parts))
+	require.Len(t, parts, 2)
+	assert.Equal(t, "input_text", parts[0].Type)
+	assert.Equal(t, "Summarize the attached document", parts[0].Text)
+	assert.Equal(t, "input_file", parts[1].Type)
+	assert.Equal(t, "document.pdf", parts[1].Filename)
+	assert.Equal(t, "data:application/pdf;base64,JVBERi0xLjQ=", parts[1].FileData)
+	assert.Empty(t, parts[1].FileID)
+}
+
+func TestChatCompletionsToResponses_FilePartFileID(t *testing.T) {
+	content := `[{"type":"file","file":{"file_id":"file-abc123"}}]`
+	req := &ChatCompletionsRequest{
+		Model: "gpt-4o",
+		Messages: []ChatMessage{
+			{Role: "user", Content: json.RawMessage(content)},
+		},
+	}
+	resp, err := ChatCompletionsToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 1)
+
+	var parts []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[0].Content, &parts))
+	require.Len(t, parts, 1)
+	assert.Equal(t, "input_file", parts[0].Type)
+	assert.Equal(t, "file-abc123", parts[0].FileID)
+	assert.Empty(t, parts[0].FileData)
+}
+
+func TestChatCompletionsToResponses_EmptyFilePartSkipped(t *testing.T) {
+	// A file part with neither file_data nor file_id carries nothing the
+	// Responses API can use; dropping it (like empty image URLs) avoids an
+	// upstream 400 on an empty input_file part.
+	content := `[{"type":"text","text":"Describe this"},{"type":"file","file":{"filename":"empty.pdf"}}]`
+	req := &ChatCompletionsRequest{
+		Model: "gpt-4o",
+		Messages: []ChatMessage{
+			{Role: "user", Content: json.RawMessage(content)},
+		},
+	}
+	resp, err := ChatCompletionsToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 1)
+
+	var parts []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[0].Content, &parts))
+	require.Len(t, parts, 1)
+	assert.Equal(t, "input_text", parts[0].Type)
+}
+
 func TestChatCompletionsToResponses_EmptyContentNeverNull(t *testing.T) {
 	// Regression for #2515: the upstream Responses API rejects an input item
 	// whose content field is JSON null. Any chat-completions message that

backend/internal/pkg/apicompat/chatcompletions_to_responses.go

@@ -378,6 +378,15 @@ func convertChatContentPartsToResponses(parts []ChatContentPart) []ResponsesCont
 					ImageURL: p.ImageURL.URL,
 				})
 			}
+		case "file":
+			if p.File != nil && (p.File.FileData != "" || p.File.FileID != "") {
+				responseParts = append(responseParts, ResponsesContentPart{
+					Type:     "input_file",
+					Filename: p.File.Filename,
+					FileData: p.File.FileData,
+					FileID:   p.File.FileID,
+				})
+			}
 		}
 	}
 	return responseParts

backend/internal/pkg/apicompat/types.go

@@ -295,9 +295,14 @@ func (i *ResponsesInputItem) UnmarshalJSON(data []byte) error {
 
 // ResponsesContentPart is a typed content part in a Responses message.
 type ResponsesContentPart struct {
-	Type     string `json:"type"` // "input_text" | "output_text" | "input_image"
+	Type     string `json:"type"` // "input_text" | "output_text" | "input_image" | "input_file"
 	Text     string `json:"text,omitempty"`
 	ImageURL string `json:"image_url,omitempty"` // data URI for input_image
+
+	// input_file fields.
+	Filename string `json:"filename,omitempty"`
+	FileData string `json:"file_data,omitempty"` // data URI
+	FileID   string `json:"file_id,omitempty"`
 }
 
 // ResponsesTool describes a tool in the Responses API.
@@ -661,9 +666,10 @@ type ChatMessage struct {
 
 // ChatContentPart is a typed content part in a multi-modal message.
 type ChatContentPart struct {
-	Type     string        `json:"type"` // "text" | "image_url"
+	Type     string        `json:"type"` // "text" | "image_url" | "file"
 	Text     string        `json:"text,omitempty"`
 	ImageURL *ChatImageURL `json:"image_url,omitempty"`
+	File     *ChatFile     `json:"file,omitempty"`
 }
 
 // ChatImageURL contains the URL for an image content part.
@@ -672,6 +678,13 @@ type ChatImageURL struct {
 	Detail string `json:"detail,omitempty"` // "auto" | "low" | "high"
 }
 
+// ChatFile contains the payload of a "file" content part (e.g. PDF input).
+type ChatFile struct {
+	Filename string `json:"filename,omitempty"`
+	FileData string `json:"file_data,omitempty"` // data URI
+	FileID   string `json:"file_id,omitempty"`
+}
+
 // ChatTool describes a tool available to the model.
 type ChatTool struct {
 	Type     string        `json:"type"` // "function"