Agents Window automation can start before models are available

Does this issue occur when all extensions are disabled?: N/A - Agents Window automations are built in.

- VS Code Version: Insiders (current main)
- OS Version: macOS

Steps to Reproduce:

1. Configure an Agents Window automation that targets a specific Agent Host session type and model.
2. Launch the Agents Window while the Agent Host and model catalog are still initializing, with the automation due for catch-up.
3. Observe that the scheduler can claim the run before the requested session type or model is available.

Expected: The scheduled run remains due and starts once the exact session target and requested model become available.

Actual: A transient startup error is recorded and the schedule advances, causing the automation run to be missed.

Related fix: #326868

cc @benvillalobos