### Steps to Reproduce:
1. Go to Helpdesk and create an SLA
2. Create a new Ticket
3. Use Studio to add the field `sla_reached`
4. Apply the SLA
5. Notice that the field is True

### Description of the issue/feature this PR addresses:
**Issue:** The `_compute_sla_reached` method in on `helpdesk_ticket.py` determines whether an SLA has been reached by checking if `exceeded_hours` is less than 0 on its `helpdesk.sla.status` records. Since every newly assigned SLA has a future deadline, `exceeded_hours` starts as negative (AKA <0), so the ticket is immediately flagged as `sla_reached = True` before any progress has been made. Conversely, a ticket that reaches its target stage after the deadline has `exceeded_hours >= 0`, so it's incorrectly flagged as `sla_reached = False` even though the SLA target was genuinely reached (it was just late).

**Solution:** Change the domain in `_compute_sla_reached` to check `reached_datetime != False` instead of `exceeded_hours < 0`, matching the field already listed in the method's `@api.depends` and the same field `_sla_reach()` sets when a ticket enters its target stage. This makes `sla_reached` reflect actual stage progression rather than a time-remaining calculation.

### Current behavior before PR: 
A newly created ticket with a pending SLA shows `sla_reached = True` and `sla_success = True` immediately upon creation, before the ticket has moved to any target stage. SImilarly, a ticket that reaches its target stage after the SLA deadline is incorrectly marked `sla_reached = False`.

### Desired behavior after PR: 
`sla_reached` is False on ticket creation and only becomes True once the ticket actually enters the SLA's target stage. Lateness continues to be tracked separately and correctly via `sla_reached_late`

opw-6361851

Forward-Port-Of: odoo/enterprise#126432