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/helpers.go

@@ -13,6 +13,11 @@ import (
 	"time"
 )
 
+const (
+	defaultMaxJSONResponseBytes int64 = 16 << 20
+	mediaMaxJSONResponseBytes   int64 = 128 << 20
+)
+
 func credential(candidate map[string]any, keys ...string) string {
 	for _, key := range keys {
 		if value, ok := candidate[key].(string); ok && strings.TrimSpace(value) != "" {
@@ -50,7 +55,12 @@ func decodeHTTPResponse(resp *http.Response) (map[string]any, error) {
 
 func decodeHTTPResponseForProtocol(resp *http.Response, protocol string) (map[string]any, *WireResponse, error) {
 	defer resp.Body.Close()
-	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 16*1024*1024))
+	maxBytes := maxJSONResponseBytesForProtocol(protocol)
+	raw, readErr := io.ReadAll(io.LimitReader(resp.Body, maxBytes+1))
+	tooLarge := int64(len(raw)) > maxBytes
+	if tooLarge {
+		raw = raw[:maxBytes]
+	}
 	wire := &WireResponse{
 		Protocol:   strings.TrimSpace(protocol),
 		StatusCode: resp.StatusCode,
@@ -60,6 +70,26 @@ func decodeHTTPResponseForProtocol(resp *http.Response, protocol string) (map[st
 	if len(raw) > 0 {
 		_ = json.Unmarshal(raw, &wire.Body)
 	}
+	if readErr != nil {
+		return nil, wire, &ClientError{
+			Code:       "response_read_error",
+			Message:    readErr.Error(),
+			StatusCode: resp.StatusCode,
+			RequestID:  requestIDFromHTTPResponse(resp),
+			Retryable:  true,
+			Wire:       wire,
+		}
+	}
+	if tooLarge {
+		return nil, wire, &ClientError{
+			Code:       "response_too_large",
+			Message:    fmt.Sprintf("upstream JSON response exceeds %d bytes", maxBytes),
+			StatusCode: resp.StatusCode,
+			RequestID:  requestIDFromHTTPResponse(resp),
+			Retryable:  false,
+			Wire:       wire,
+		}
+	}
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
 		return nil, wire, &ClientError{
 			Code:       statusCodeName(resp.StatusCode),
@@ -82,6 +112,15 @@ func decodeHTTPResponseForProtocol(resp *http.Response, protocol string) (map[st
 	return out, wire, nil
 }
 
+func maxJSONResponseBytesForProtocol(protocol string) int64 {
+	switch strings.TrimSpace(protocol) {
+	case ProtocolGeminiGenerateContent, ProtocolOpenAIImages:
+		return mediaMaxJSONResponseBytes
+	default:
+		return defaultMaxJSONResponseBytes
+	}
+}
+
 func compatibleResponseHeaders(source http.Header) map[string][]string {
 	if len(source) == 0 {
 		return nil