[18.0][FIX] helpdesk_mgmt_sla: use _get_resource_calendar() with fallback for SLA calculations

[xaviedoanhduy]

Currently, SLA computations (_compute_sla_data, _compute_deadline, _stage_recompute) access the calendar directly via ticket_id.team_id.resource_calendar_id. This approach causes errors when:

• The ticket has no assigned team (team_id = False), or
• The SLA is running while the ticket is being updated.

These situations lead to singleton or NoneType errors, causing SLA computation to crash.
Screencast from 2025-11-17 16-09-08.webm

Introduce _get_resource_calendar() to safely retrieve a calendar for SLA computations:

• Fallback order: user → team → ticket company → default company calendar.
• Benefits: prevents singleton or None errors when a ticket has no team or user, and prioritizes the user’s working hours for SLA.