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/database/queries/workspaces.sql

@@ -758,7 +758,12 @@ WHERE
 			provisioner_jobs.completed_at IS NOT NULL AND
 			(@now :: timestamptz) - provisioner_jobs.completed_at > (INTERVAL '1 millisecond' * (templates.failure_ttl / 1000000))
 		)
-	) AND workspaces.deleted = 'false';
+	)
+  	AND workspaces.deleted = 'false'
+  	-- Prebuilt workspaces (identified by having the prebuilds system user as owner_id)
+	-- should not be considered by the lifecycle executor, as they are handled by the
+	-- prebuilds reconciliation loop.
+  	AND workspaces.owner_id != 'c42fdf75-3097-471c-8c33-fb52454d81c0'::UUID;
 
 -- name: UpdateWorkspaceDormantDeletingAt :one
 UPDATE