RE: Saving lead entity without mandatory fields filled
Sankalp, as mentioned before, if you want to have that fields validation when pressing "Qualify" button, you need to create a synchronous PreValidation Plug-in on "QualifyLead" message, that will fire only when clicking "Qualify" button.
Then, inside the Plug-in logic, you can check if the fields are populated and, if they are empty, block Lead qualification and display an error message to the user.
Please mark the answer as verified if helpful.