refactor(db): streamline model imports and remove unused model exports

- Removed the export of models from the database connection module to simplify the structure.
- Updated various files to import models directly from the new centralized models module.
- Ensured consistency across the codebase by replacing mongoose model references with the new import paths.

api/models/ToolCall.js

@@ -1,6 +1,6 @@
 const mongoose = require('mongoose');
 
-const ToolCall = mongoose.models.ToolCall;
+const ToolCall = require('~/db/models').ToolCall;
 
 /**
  * Create a new tool call