fix: 修复Claude OAuth token交换时authorization code解析错误

原代码中 `parts` 变量被创建但从未使用，导致 `len(parts) == 0`
永远为 true，使得即使成功从 `code#state` 格式中分割出 authCode，
最后也会被覆盖为原始的完整字符串。

这导致传递给Claude Token端点的code包含了 `#state` 部分，
Claude返回 "Invalid 'code' in request" 错误。

backend/internal/repository/claude_oauth_service.go

@@ -7,6 +7,7 @@ import (
 	"log"
 	"net/http"
 	"net/url"
+	"strings"
 	"time"
 
 	"sub2api/internal/pkg/oauth"
@@ -139,20 +140,12 @@ func (s *claudeOAuthService) GetAuthorizationCode(ctx context.Context, sessionKe
 func (s *claudeOAuthService) ExchangeCodeForToken(ctx context.Context, code, codeVerifier, state, proxyURL string) (*oauth.TokenResponse, error) {
 	client := createReqClient(proxyURL)
 
+	// Parse code which may contain state in format "authCode#state"
 	authCode := code
 	codeState := ""
-	if len(code) > 0 {
-		parts := make([]string, 0, 2)
-		for i, part := range []rune(code) {
-			if part == '#' {
-				authCode = code[:i]
-				codeState = code[i+1:]
-				break
-			}
-		}
-		if len(parts) == 0 {
-			authCode = code
-		}
+	if idx := strings.Index(code, "#"); idx != -1 {
+		authCode = code[:idx]
+		codeState = code[idx+1:]
 	}
 
 	reqBody := map[string]any{