chore: fix 500 response err log

[zhaohuabing]

Before:

2025-10-27T12:42:34.100Z	INFO	gateway-api	gatewayapi/route.go:294	setting 500 direct response in routes due to errors in processing destinations	{"runner": "gateway-api", "routes": ["httproute/default/backend/rule/0/match/0"], "error": null}

After:

2025-10-27T13:15:17.501Z	INFO	gateway-api	gatewayapi/route.go:294	setting 500 direct response in routes due to errors in processing destinations	{"runner": "gateway-api", "routes": ["httproute/default/backend/rule/0/match/0"], "error": "ClientCertificateRef Secret is not located in the same namespace as Backend. Secret namespace:  does not match Backend namespace: default"}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.06%. Comparing base (4fd1fc1) to head (4802ae6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7345      +/-   ##
==========================================
+ Coverage   72.01%   72.06%   +0.05%     
==========================================
  Files         230      230              
  Lines       33370    33370              
==========================================
+ Hits        24031    24049      +18     
+ Misses       7591     7575      -16     
+ Partials     1748     1746       -2     

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