test failed in CI: test_migration_completed_source_destroyed_can_unwind

This test failed on a CI run on https://github.com/oxidecomputer/omicron/pull/8421:

https://github.com/oxidecomputer/omicron/pull/8421/checks?check_run_id=46436575981

Log showing the specific test failure:

https://buildomat.eng.oxide.computer/wg/0/details/01K0QXDVR8RSGDZYTB7SRN9ABS/PVjenh9an23rHPQ7wXNztDHjijSTCH3RiCN7IXstGrwWviT8/01K0QXEFADYPT7JY0E9J0AP1JK#S7242

Excerpt from the log showing the failure:

7255	2025-07-22T02:58:40.792Z	  stderr ───
7256	2025-07-22T02:58:40.792Z	    log file: /var/tmp/omicron_tmp/omicron_nexus-a6e67636cf780741-test_migration_completed_source_destroyed_can_unwind.20150.0.log
7257	2025-07-22T02:58:40.792Z	    note: configured to log to "/var/tmp/omicron_tmp/omicron_nexus-a6e67636cf780741-test_migration_completed_source_destroyed_can_unwind.20150.0.log"
7258	2025-07-22T02:58:40.793Z	    DB URL: postgresql://root@[::1]:59248/omicron?sslmode=disable
7259	2025-07-22T02:58:40.793Z	    DB address: [::1]:59248
7260	2025-07-22T02:58:40.793Z	    log file: /var/tmp/omicron_tmp/omicron_nexus-a6e67636cf780741-test_migration_completed_source_destroyed_can_unwind.20150.2.log
7261	2025-07-22T02:58:40.793Z	    note: configured to log to "/var/tmp/omicron_tmp/omicron_nexus-a6e67636cf780741-test_migration_completed_source_destroyed_can_unwind.20150.2.log"
7262	2025-07-22T02:58:40.793Z	    log file: /var/tmp/omicron_tmp/omicron_nexus-a6e67636cf780741-test_migration_completed_source_destroyed_can_unwind.20150.3.log
7263	2025-07-22T02:58:40.793Z	    note: configured to log to "/var/tmp/omicron_tmp/omicron_nexus-a6e67636cf780741-test_migration_completed_source_destroyed_can_unwind.20150.3.log"
7264	2025-07-22T02:58:40.793Z	
7265	2025-07-22T02:58:40.793Z	    thread 'app::sagas::instance_update::test::test_migration_completed_source_destroyed_can_unwind' panicked at sled-agent/src/sim/collection.rs:274:65:
7266	2025-07-22T02:58:40.793Z	    called `Result::unwrap()` on an `Err` value: InternalError { internal_message: "Communication Error: error sending request for url (http://127.0.0.1:41246/vmms/5a407a30-9b19-4ca6-b88d-580ab15f2af8)" }

Start with app::sagas::instance_update::test::test_migration_completed_source_destroyed_can_unwind and inspect sled-agent/src/sim/collection.rs around line 274. Reproduce the CI failure using the linked check and log, then trace the reported communication error; done means the test no longer panics in CI.