feat: add workspace restart functionality to API (#25757)

This models restart as durable orchestration of existing stop and
start workspace builds instead of adding a new restart transition.
Keeping restart as two existing transitions preserves the current
build/provisioner model.

The child start build is created only after the parent stop build
succeeds, rather than being inserted immediately in a pending
state. That keeps `workspace_builds` aligned with actual
provisioner-ready work and avoids introducing a second
pending-build lifecycle that the provisioner and build acquisition
paths would need to understand.

Refs: https://linear.app/codercom/issue/PLAT-143

coderd/database/migrations/testdata/fixtures/000540_workspace_build_orchestrations.up.sql

@@ -0,0 +1,21 @@
+INSERT INTO workspace_build_orchestrations (
+	id,
+	created_at,
+	updated_at,
+	workspace_id,
+	parent_build_id,
+	child_transition
+)
+SELECT
+	'4e983a68-9b8a-4d4e-a4d6-5f2dd73551c2'::uuid,
+	NOW(),
+	NOW(),
+	workspace_id,
+	id,
+	'start'::workspace_transition
+FROM
+	workspace_builds
+ORDER BY
+	created_at, id
+LIMIT 1
+ON CONFLICT DO NOTHING;