Hotfix: Fix list.strip() error in config parser validation

🐛 Critical Bug Fix:
- Fixed 'list' object has no attribute 'strip' error
- Error occurred in _validate_parsed_config() at line 847
- use_backend_rules is now a list, not a string

🔧 Technical Details:
- Changed from: frontend.use_backend_rules.strip()
- Changed to: bool(frontend.use_backend_rules)
- Simple boolean check works for both list and None types

✅ Impact:
- Bulk import parsing now works without errors
- Config validation properly handles list-based use_backend_rules
- All warning messages display correctly

Error was:
  'list' object has no attribute 'strip'
  at _validate_parsed_config line 847

Fix applied:
  Line 847-848: Use bool() instead of .strip() for list validation

backend/utils/haproxy_config_parser.py

@@ -844,7 +844,8 @@ class HAProxyConfigParser:
             
             # Check if frontend has any backend routing
             has_default_backend = frontend.default_backend is not None
-            has_use_backend_rules = frontend.use_backend_rules is not None and frontend.use_backend_rules.strip()
+            # CRITICAL FIX: use_backend_rules is now a list, not a string
+            has_use_backend_rules = bool(frontend.use_backend_rules)
             
             if not has_default_backend and not has_use_backend_rules:
                 # Frontend has no routing - check if it has redirect rules