Fix/Test cases coverage

[Diya-0816]

This PR enhances test coverage and validation for user-related operations by introducing new test cases and enforcing stricter input checks.

Changes Implemented:

• Added test cases for:
  • Username validation (min length, allowed characters, duplicate check)
  • Password validation (basic format checks if applicable)
  • Profile field edge cases (empty update payloads)
• Enforced that profile update requests must include at least one non-null field using a @root_validator.
• Ensured invalid update requests return 422 Unprocessable Entity.
• Confirmed duplicate usernames return a 409 Conflict response.
• Ensured overall test suite passes and improves coverage threshold.