fix(media): 支持大尺寸图像响应并补齐迁移契约

- 为图像协议放宽 JSON 响应上限并显式处理读取与超限错误
- 合并基础模型与平台能力，避免运行时丢失分辨率和比例约束
- 固化稳定 Gemini 图像模型的能力、计价和 preview 兼容别名

apps/api/internal/clients/wire_response_test.go

@@ -46,6 +46,46 @@ func TestDecodeHTTPResponseForProtocolCapturesOfficialErrorWire(t *testing.T) {
 	}
 }
 
+func TestDecodeHTTPResponseForProtocolAllowsLargeImagePayload(t *testing.T) {
+	payload := `{"data":"` + strings.Repeat("a", int(defaultMaxJSONResponseBytes)) + `"}`
+	response := &http.Response{
+		StatusCode: http.StatusOK,
+		Status:     "200 OK",
+		Header:     http.Header{"Content-Type": {"application/json"}},
+		Body:       io.NopCloser(strings.NewReader(payload)),
+	}
+
+	result, wire, err := decodeHTTPResponseForProtocol(response, ProtocolGeminiGenerateContent)
+	if err != nil {
+		t.Fatalf("large Gemini image response failed: %v", err)
+	}
+	if got, _ := result["data"].(string); len(got) != int(defaultMaxJSONResponseBytes) {
+		t.Fatalf("large Gemini image payload length = %d", len(got))
+	}
+	if wire == nil || len(wire.RawJSON) != len(payload) {
+		t.Fatalf("large Gemini wire payload was truncated: %+v", wire)
+	}
+}
+
+func TestDecodeHTTPResponseForProtocolRejectsOversizedDefaultPayload(t *testing.T) {
+	payload := `{"data":"` + strings.Repeat("a", int(defaultMaxJSONResponseBytes)) + `"}`
+	response := &http.Response{
+		StatusCode: http.StatusOK,
+		Status:     "200 OK",
+		Header:     http.Header{"Content-Type": {"application/json"}},
+		Body:       io.NopCloser(strings.NewReader(payload)),
+	}
+
+	_, _, err := decodeHTTPResponseForProtocol(response, ProtocolOpenAIResponses)
+	if err == nil {
+		t.Fatal("expected oversized default response to fail")
+	}
+	clientErr, ok := err.(*ClientError)
+	if !ok || clientErr.Code != "response_too_large" {
+		t.Fatalf("unexpected oversized response error: %T %v", err, err)
+	}
+}
+
 func TestGeminiNativeStreamPreservesOfficialEvents(t *testing.T) {
 	var requestedPath string
 	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {