Close the harness command client before Windows removes its state dir

Since #2550, three command identity tests fail on Windows x64:
TempDir cleanup cannot delete operation-receipts.db because the agent's
command client still holds it open. Those tests never call run, so
nothing closes the client that New built, and Windows refuses to delete
an open file. Other platforms delete it anyway.

Stop and close the command client in a harness cleanup. It is
registered after the state TempDir, so it runs first, and Close is
idempotent for the tests that do run the agent. Production already
closes the client when Run exits or an identity supersedes it.

internal/hostagent/command_client_test.go

@@ -601,6 +601,10 @@ func newCommandIdentityHarness(t *testing.T, presentedID string, interval time.D
 	agent.httpClient = h.server.Client()
 	agent.trimmedPulseURL = h.server.URL
 	h.agent = agent
+	// Run closes the command client on exit, but several tests never call
+	// run. Close it here, before the state TempDir is removed: Windows cannot
+	// delete the open operation-receipts.db.
+	t.Cleanup(func() { agent.stopCommandClient(true) })
 	return h
 }