fix: clear consumed ephemeral registration state (#1256)

Ephemeral registrations are single-use, but the daemon kept `.runner` after the task, so a restarted container failed `Declare` with `unregistered runner` forever.

An ephemeral runner now removes its registration file as soon as it fetches a task, so a container killed mid-job registers afresh on restart. It also removes the file when Gitea answers `unregistered runner` at startup or while polling. A proxy's 401 does not count. Persistent runners keep their file, because Gitea sends the same reply for a token mismatch.

Closes https://gitea.com/gitea/runner/issues/818

---------

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1256
Reviewed-by: silverwind <[email protected]>

e2e/ephemeral_test.go

@@ -22,7 +22,7 @@ func testEphemeralRunner(t *testing.T) {
 	case <-time.After(5 * time.Second):
 		t.Fatal("ephemeral runner was not deleted")
 	}
-	if !poller.Unregistered() {
+	if !poller.RegistrationRejected() {
 		t.Fatal("ephemeral runner stopped without being deleted")
 	}
 }