Refresh token not updated in user document after issuing new one in refreshAccessToken method

Description

In the refreshAccessToken method, a new refresh token is generated, but it is not stored back in the user's record in the database.

This results in:

• Old refresh tokens still remaining valid
• New refresh tokens not being persisted
• Potential security issues if the old token is used again (replay attacks)

Steps to Reproduce

1. Login and get a refresh token.
2. Call the refresh endpoint to generate new tokens.
3. Check the database — the user.refreshToken is not updated.

Suggested Fix

After generating a new refresh token, update the user's document:

user.refreshToken = newRefreshToken;
await user.save({ validateBeforeSave: false });