fix: rollback DB session after failed tool execution in chat agent

When a tool like query_database fails (e.g., bad column name), the
SQLAlchemy session enters a failed transaction state. Subsequent
operations (like saving the assistant response) then also fail with
InFailedSQLTransactionError.

Fix: rollback the session in the except block of _execute_tool().
Also improved query_database tool description with correct column
names (category_key not category) to help the AI write valid SQL.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/chat_service.py

@@ -219,7 +219,7 @@ TOOLS = [
         "type": "function",
         "function": {
             "name": "query_database",
-            "description": "Execute a read-only SQL SELECT query against the database. Results are automatically filtered to the current tenant. Tables: orders, order_lines, products, cad_files, materials, material_aliases, output_types, media_assets, render_templates.",
+            "description": "Execute a read-only SQL SELECT query against the database. Key columns: products(id, name, pim_id, category_key, cad_file_id, is_active), orders(id, order_number, status, tenant_id), order_lines(id, order_id, product_id, render_status, material_override, render_overrides). Use :tenant_id parameter for tenant filtering. Category is 'category_key' not 'category'.",
             "parameters": {
                 "type": "object",
                 "properties": {
@@ -292,8 +292,13 @@ async def _execute_tool(
         else:
             return json.dumps({"error": f"Unknown tool: {name}"})
     except Exception as exc:
-        logger.exception("Tool execution failed: %s(%s)", name, arguments)
-        return json.dumps({"error": str(exc)})
+        logger.warning("Tool execution failed: %s(%s): %s", name, arguments, exc)
+        # Rollback the DB session so subsequent operations still work
+        try:
+            await db.rollback()
+        except Exception:
+            pass
+        return json.dumps({"error": str(exc)[:500]})
 
 
 async def _tool_list_orders(db: AsyncSession, tenant_id: str, status: str = "", limit: int = 10) -> str: