fix(sse): 修复非标准 SSE 格式解析问题

部分上游 API 返回的 SSE 格式不符合标准规范：
- 标准格式: `data: {...}`（冒号后有空格）
- 非标准格式: `data:{...}`（冒号后无空格）

使用预编译正则 `^data:\s*` 统一处理两种格式。

backend/internal/service/account_test_service.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -21,6 +22,10 @@ import (
 	"github.com/google/uuid"
 )
 
+// sseDataPrefix matches SSE data lines with optional whitespace after colon.
+// Some upstream APIs return non-standard "data:" without space (should be "data: ").
+var sseDataPrefix = regexp.MustCompile(`^data:\s*`)
+
 const (
 	testClaudeAPIURL   = "https://api.anthropic.com/v1/messages"
 	testOpenAIAPIURL   = "https://api.openai.com/v1/responses"
@@ -412,11 +417,11 @@ func (s *AccountTestService) processClaudeStream(c *gin.Context, body io.Reader)
 		}
 
 		line = strings.TrimSpace(line)
-		if line == "" || !strings.HasPrefix(line, "data: ") {
+		if line == "" || !sseDataPrefix.MatchString(line) {
 			continue
 		}
 
-		jsonStr := strings.TrimPrefix(line, "data: ")
+		jsonStr := sseDataPrefix.ReplaceAllString(line, "")
 		if jsonStr == "[DONE]" {
 			s.sendEvent(c, TestEvent{Type: "test_complete", Success: true})
 			return nil
@@ -466,11 +471,11 @@ func (s *AccountTestService) processOpenAIStream(c *gin.Context, body io.Reader)
 		}
 
 		line = strings.TrimSpace(line)
-		if line == "" || !strings.HasPrefix(line, "data: ") {
+		if line == "" || !sseDataPrefix.MatchString(line) {
 			continue
 		}
 
-		jsonStr := strings.TrimPrefix(line, "data: ")
+		jsonStr := sseDataPrefix.ReplaceAllString(line, "")
 		if jsonStr == "[DONE]" {
 			s.sendEvent(c, TestEvent{Type: "test_complete", Success: true})
 			return nil