fix: exclude prebuilt workspaces from lifecycle executor (#18762)

## Description

This PR updates the lifecycle executor to explicitly exclude prebuilt
workspaces from being considered for lifecycle operations such as
`autostart`, `autostop`, `dormancy`, `default TTL` and `failure TTL`.

Prebuilt workspaces (i.e., those owned by the prebuild system user) are
handled separately by the prebuild reconciliation loop. Including them
in the lifecycle executor could lead to unintended behavior such as
incorrect scheduling or state transitions.

## Changes

* Updated the lifecycle executor query
`GetWorkspacesEligibleForTransition` to exclude workspaces with
`owner_id = 'c42fdf75-3097-471c-8c33-fb52454d81c0'` (prebuilds).
* Added tests to verify prebuilt workspaces are not considered in:
  * Autostop
  * Autostart
  * Default TTL
  * Dormancy
  * Failure TTL

Fixes: https://github.com/coder/coder/issues/18740
Related to: https://github.com/coder/coder/issues/18658

coderd/schedule/autostart.go

@@ -33,6 +33,8 @@ func NextAutostart(at time.Time, wsSchedule string, templateSchedule TemplateSch
 	return zonedTransition, allowed
 }
 
+// NextAllowedAutostart returns the next valid autostart time after 'at', based on the workspace's
+// cron schedule and the template's allowed days. It searches up to 7 days ahead to find a match.
 func NextAllowedAutostart(at time.Time, wsSchedule string, templateSchedule TemplateScheduleOptions) (time.Time, error) {
 	next := at