On-Call Escalation Rule "Notify" step still forces Call as default channel - no way to prioritize SMS/Email (Reopening #2862)

This issue was previously reported and closed as #2862, but the underlying problem still exists in the current version. Reopening with updated context based on the On-Call Readiness page, which is a genuinely helpful addition — but it highlights this gap further.

On-Call Policy → Escalation Rule → "Notify" step lets us select On-Call Schedules, Teams, and Users to notify, but there's no way to configure the order or priority of notification channels (SMS, Email, Call) for that rule. Regardless of what's configured, the system defaults to calling the on-call person directly, rather than trying other channels first.

The On-Call Readiness page (Policies → Readiness) confirms responders have multiple channels available (visible under "Channels" as colored bars per responder), but there's no way to control which channel is tried first at the rule level — it always defaults to Call.

Describe the solution you'd like On the Escalation Rule "Notify" step, allow configuring the order of notification channels per rule — for example:

1. First attempt: SMS
2. Second attempt (if unacknowledged): Email
3. Third attempt (if still unacknowledged): Call

This way, Call is only used as a fallback after SMS/Email have been tried, instead of being the default first (and often only) channel used.

Additional context This builds directly on the On-Call Readiness feature, which is very useful for confirming responders are reachable — but readiness alone doesn't help if the escalation rule itself always defaults to Call regardless of what channels are available or preferred. Giving explicit control over channel order per rule would let teams reduce unnecessary phone calls and rely on SMS/Email first where appropriate, while still falling back to Call when needed.

Screenshots (attach: closed issue #2862 for reference/history; Edit Escalation Rule → Notify step showing On-Call Schedules/Teams/Users selection with no channel-order option; On-Call Readiness page showing responders and their available Channels)