fix: prevent invitation of disabled users with clear validation message

[devdattatalele]

Summary

Fixes user invitation issues described in #2473 by adding proper backend validation for disabled users.

Changes Made

• Backend Validation: Added check in helpdesk/api/agent.py to detect disabled users before creating invitations
• Clear Error Messages: Users now receive specific feedback when attempting to invite disabled users
• Prevention of Errors: Stops the system from creating invalid invitation states

Problem Solved

Before: Inviting a disabled user would cause unclear errors or system issues
After: Clear validation message: "User {email} is disabled. Please enable the user first to invite them."

Testing

• Tested with disabled user accounts
• Verified error message displays correctly
• Confirmed no invitation records created for disabled users
• Validated existing enabled user invitations still work

Related Issues

Addresses part 1 of issue #2473

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

Additional Notes

This change focuses on the invitation validation issue. The user deletion and password reset issues mentioned in #2473 may require separate PRs for targeted fixes.

[devdattatalele]

import frappe
+from frappe import _

@frappe.whitelist()
def sent_invites(emails, send_welcome_mail_to_user=True):
    for email in emails:
        if frappe.db.exists("User", email):
            user = frappe.get_doc("User", email)
+            if not user.enabled:
+                frappe.throw(
+                    _("User with email {0} is disabled. Please enable the user first to invite them.").format(email),
+                    frappe.ValidationError
+                )
        else:
            user = frappe.get_doc(
                {"doctype": "User", "email": email, "first_name": email.split("@")[0]}
            ).insert()

            if send_welcome_mail_to_user:
                user.send_welcome_mail_to_user()