test failed in CI: `app::sagas::instance_update::test::test_migration_target_completed_can_unwind`

This test failed on a CI run on #9556:

https://github.com/oxidecomputer/omicron/pull/9556/checks?check_run_id=58893467030

Log showing the specific test failure:

https://buildomat.eng.oxide.computer/wg/0/details/01KD98PC9VHGP50Z14Y73FHVT5/rHftY69SxSfewj1dbX7xKm8YhSJXop3X4wxWe2jUiArGDbHx/01KD98Q5603VY79GXVF5YG0CRA#S7811

Excerpt from the log showing the failure:

stdout ───
  running 1 test
  test app::sagas::instance_update::test::test_migration_target_completed_can_unwind ... FAILED
  failures:
  failures:
      app::sagas::instance_update::test::test_migration_target_completed_can_unwind
  test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 210 filtered out; finished in 39.93s
  
stderr ───
  log file: /var/tmp/omicron_tmp/omicron_nexus-def80fee552906de-test_migration_target_completed_can_unwind.23627.0.log
  note: configured to log to "/var/tmp/omicron_tmp/omicron_nexus-def80fee552906de-test_migration_target_completed_can_unwind.23627.0.log"
  DB URL: postgresql://root@[::1]:55652/omicron?sslmode=disable
  DB address: [::1]:55652
  log file: /var/tmp/omicron_tmp/omicron_nexus-def80fee552906de-test_migration_target_completed_can_unwind.23627.2.log
  note: configured to log to "/var/tmp/omicron_tmp/omicron_nexus-def80fee552906de-test_migration_target_completed_can_unwind.23627.2.log"
  log file: /var/tmp/omicron_tmp/omicron_nexus-def80fee552906de-test_migration_target_completed_can_unwind.23627.3.log
  note: configured to log to "/var/tmp/omicron_tmp/omicron_nexus-def80fee552906de-test_migration_target_completed_can_unwind.23627.3.log"
  thread 'app::sagas::instance_update::test::test_migration_target_completed_can_unwind' (2) panicked at sled-agent/src/sim/collection.rs:274:65:
  called `Result::unwrap()` on an `Err` value: InternalError { internal_message: "Communication Error: error sending request for url (http://127.0.0.1:35432/vmms/79b945d1-2a50-405d-a93e-f0b88710a7ca)" }
  stack backtrace:
...

Start with app::sagas::instance_update::test::test_migration_target_completed_can_unwind and the CI log linked in the issue. Inspect sled-agent/src/sim/collection.rs at line 274, then reproduce the test failure and trace the reported communication error. Done means the test no longer fails unexpectedly in CI.