feat: get user testimonials

[Laban254]

Description

This PR introduces an API endpoint to retrieve all testimonials associated with a specific user.

Key Changes:

• Added GET /api/v1/testimonials/user/{user_id} endpoint.
• Validated user_id parameter to ensure it exists and is correctly formatted.
• Wrote unit tests.

Related Issue

#372

How Has This Been Tested?

• Manually tested the endpoint using Postman with valid and invalid inputs.
• Ran unit tests covering:
  • Successful retrieval of testimonials.
  • Requests with an invalid or non-existing user_id.
  • Users with no testimonials.
• Ensured all existing tests pass.

**Screenshots **

Success

Invalid User ID

Unavaillable User

Types of Changes

• New feature (non-breaking change that adds functionality)
• Bug fix (non-breaking change that fixes an issue)
• Breaking change (fix or feature that affects existing functionality)

Checklist

• My code follows the project's coding style.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests pass.