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/Agent.spec.js

@@ -21,7 +21,7 @@ const {
   removeAgentResourceFiles,
 } = require('./Agent');
 
-const Agent = mongoose.models.Agent;
+const Agent = require('~/db/models').Agent;
 
 describe('Agent Resource File Operations', () => {
   let mongoServer;