Fix: Improved slug validation and error handling for courses

[AliyanA1]

#6087 This PR fixes an issue where updating a course slug to a duplicate value caused a generic "Internal Server Error." The changes include:

1. Added Slug Validation in Course Model:

   • Ensures slug is unique with a clear error message:
     "Slug must be unique. Another course with this slug already exists."

2. Improved Error Handling in update Method:

   • The courses_controller.rb now validates updates and provides meaningful error messages to users when a slug conflict occurs.
   • Returns a 422 Unprocessable Entity status with detailed validation messages.

[ragesoss]

Please add before/after screenshots demonstrating the change.