feat: add validation methods for Git link and contact presence

- Added `validate` method to ensure each Student object has a Git link and at least one contact method (phone, telegram, or email).
- Implemented `git_present?` method to check the presence of a Git link.
- Implemented `contact_present?` method to check the presence of at least one contact method.
- Modified the constructor to call `validate` during object initialization.

lab2/student.rb

@@ -46,6 +46,21 @@ class Student
 
         @git = args[:git]
         raise ArgumentError, "Invalid git format: #{@git}" unless Student.valid_git?(@git)
+
+        validate
+    end
+
+    def git_present?
+        !@git.nil? && !@git.empty?
+    end
+
+    def contact_present?
+        !(@phone.nil? || @phone.empty?) || !(@telegram.nil? || @telegram.empty?) || !(@email.nil? || @email.empty?)
+    end
+
+    def validate
+        raise ArgumentError, "Git link is required" unless git_present?
+        raise ArgumentError, "At least one contact (phone, telegram, or email) is required" unless contact_present?
     end
 
     def to_s