Enhance V3 CRUD: Implement Error Bubbling and Dry-Run Validation.

- Updated app/db_sql.py to capture SQL exceptions in thread-local storage for later retrieval.
- Implemented format_db_error() in app/lib_api_crud_v3.py to clean up raw MariaDB error strings.
- Added POST /v3/crud/{obj_type}/validate endpoint for dry-run payload validation.
- Updated main and nested routers to bubble up validation and database errors into the response 'meta.details' field.
- Added tests/test_v3_error_bubbling.py to verify formatting logic.

app/lib_api_crud_v3.py

@@ -1,11 +1,28 @@
 from typing import Any, Dict, Optional
 import json
 import logging
+import re
 
 from app.lib_general_v3 import AccountContext, StatusFilterParams
 
 log = logging.getLogger(__name__)
 
+def format_db_error(raw_error: str) -> str:
+    """
+    Parses raw SQLAlchemy/MariaDB errors into user-friendly strings.
+    """
+    if not raw_error:
+        return ""
+    
+    # Standard MariaDB pattern: (code, "message")
+    match = re.search(r'\(\d+,\s*["\'](.*?)["\']\s*\)', raw_error)
+    if match:
+        return match.group(1).strip()
+    
+    # Fallback: remove all (parenthesized) blocks which often contain codes
+    clean = re.sub(r'\(.*?\)', '', raw_error)
+    return clean.strip()
+
 def check_account_access(sql_result: Any, account: AccountContext, obj_name: str = None) -> bool:
     """
     Enforce Multi-Tenant Data Isolation.