add input validation to mentor endpoints

[ProgrammerSONGBIT]

Added validation helpers for mentor input and integrated them into the registration process. Ensured that name and email fields are validated and trimmed before creating or updating mentor records.
Changes =

• Add ValidateMentorInput() helper for consistent validation
• Add IsValidEmail() helper for email format validation
• Validate name: required, 2-100 characters
• Validate email: required, valid format, max 255 characters
• Sanitize input with strings.TrimSpace()
• Return 400 Bad Request for validation failures
• Log validation errors for debugging

[ProgrammerSONGBIT]

LGTM
I will remove the comments if you suggest ?

[ProgrammerSONGBIT]

@dipamsen I added validation helpers for mentor input and integrated them into the registration process.