[IMP] real_estate: improve property appointment creation flow

[dhgoy-odoo]

Appointment workflow:

• Add an "Appointments" smart button on the property form view.
• Display the total number of appointments linked to the property.
• Open the property's related appointments when clicking on the smart button.
• Redirect users to the created appointment records after clicking on the
  "Create Visit Link" button.

Demo data and test update:

• Since the Agent field is mandatory for appointment creation, add x_agent_id in
  the demo property records.
• Add x_agent_id in the property test data since the agent is required for visit
  link creation.

Bug fix:

Issue:

• The "Create Appointment & Link" action was creating duplicate appointments
  and links even for properties whose link was already generated.

Steps to reproduce:

• Open the property list view.
• Select properties whose appointments are already created, and a link is
  generated.
• Click on Actions -> "Create Appointment & Link".

Solution:

• Skip properties whose link has already been generated when creating
  appointments/link from the server action menu.
• Generate visit links and appointments only for selected properties whose links
  are not already generated, and show a validation message when all selected
  Properties already have existing links.

Task ID-6217777

[robodoo]