fix: prevent activity bump for prebuilt workspaces (#19263)

## Description

This PR ensures that activity-based deadline extensions ("activity
bumping") are not applied to prebuilt workspaces. Prebuilds are managed
by the reconciliation loop and must not have `deadline` or
`max_deadline` values set or extended, as they are not part of the
regular lifecycle executor path.

## Changes

- Update `ActivityBumpWorkspace` SQL query to discard prebuilt
workspaces
- Update application layer to avoid calling activity bump logic on
prebuilt workspaces

Related with: 
* Issue: https://github.com/coder/coder/issues/18898
* PR: https://github.com/coder/coder/pull/19252

coderd/database/queries/activitybump.sql

@@ -22,7 +22,7 @@ WITH latest AS (
 				-- be as if the workspace auto started at the given time and the
 				-- original TTL was applied.
 				--
-				-- Sadly we can't define `activity_bump_interval` above since
+				-- Sadly we can't define 'activity_bump_interval' above since
 				-- it won't be available for this CASE statement, so we have to
 				-- copy the cast twice.
 				WHEN NOW() + (templates.activity_bump / 1000 / 1000 / 1000 || ' seconds')::interval > @next_autostart :: timestamptz
@@ -52,7 +52,11 @@ WITH latest AS (
 		ON workspaces.id = workspace_builds.workspace_id
 	JOIN templates
 		ON templates.id = workspaces.template_id
-	WHERE workspace_builds.workspace_id = @workspace_id::uuid
+	WHERE
+		workspace_builds.workspace_id = @workspace_id::uuid
+		-- Prebuilt workspaces (identified by having the prebuilds system user as owner_id)
+		-- are managed by the reconciliation loop and not subject to activity bumping
+	  	AND workspaces.owner_id != 'c42fdf75-3097-471c-8c33-fb52454d81c0'::UUID
 	ORDER BY workspace_builds.build_number DESC
 	LIMIT 1
 )