fix: Resolve UnboundLocalError for 're' module in technical system detection

ISSUE: UnboundLocalError at line 5387: cannot access local variable 're' where it is not associated with a value

ROOT CAUSE:
- Multiple uses of 're.search()' in has_your_system_pattern_rag detection (lines 5387-5393)
- Python detects 're' as a local variable if it's assigned anywhere in the function
- Previous fix only addressed line 3848, but line 5387 also uses 're' module

FIX:
- Import 're' module explicitly as 'regex_module_rag' within the detection block
- Replace all 're.search()' calls with 'regex_module_rag.search()' in has_your_system_pattern_rag
- This ensures no scope conflicts and prevents UnboundLocalError

BENEFITS:
- Fixes chat endpoint crash for technical system questions
- Prevents UnboundLocalError in all code paths
- Consistent with previous fix at line 3848

Author: anhmtk

backend/api/routers/chat_router.py

@@ -5381,16 +5381,18 @@ def truncate_text(text: str, max_tokens: int) -> str:
                 ])
                 # CRITICAL: Improved detection for "your system" questions
                 # Match patterns like "in your system", "your system", "system you", etc.
+                # CRITICAL: Import re module explicitly to avoid UnboundLocalError
+                import re as regex_module_rag
                 has_your_system_pattern_rag = (
                     "your system" in question_lower_rag or
                     "in your system" in question_lower_rag or
-                    re.search(r'\bin\s+your\s+system\b', question_lower_rag) or  # "in your system"
-                    re.search(r'\byour\s+\w+\s+system\b', question_lower_rag) or  # "your X system"
-                    re.search(r'\bsystem\s+\w+\s+you\b', question_lower_rag) or  # "system X you"
-                    re.search(r'\bsystem\s+you\b', question_lower_rag) or  # "system you"
+                    regex_module_rag.search(r'\bin\s+your\s+system\b', question_lower_rag) or  # "in your system"
+                    regex_module_rag.search(r'\byour\s+\w+\s+system\b', question_lower_rag) or  # "your X system"
+                    regex_module_rag.search(r'\bsystem\s+\w+\s+you\b', question_lower_rag) or  # "system X you"
+                    regex_module_rag.search(r'\bsystem\s+you\b', question_lower_rag) or  # "system you"
                     "bạn" in question_lower_rag and "hệ thống" in question_lower_rag or
                     "của bạn" in question_lower_rag or
-                    re.search(r'\bhệ\s+thống\s+của\s+bạn\b', question_lower_rag)  # "hệ thống của bạn"
+                    regex_module_rag.search(r'\bhệ\s+thống\s+của\s+bạn\b', question_lower_rag)  # "hệ thống của bạn"
                 )
                 # CRITICAL: Also check if this was already detected as StillMe query (from earlier detection)
                 # This ensures technical questions about "your system" are properly flagged for retry logic