test failed in CI: test_disk_move_between_instances

(This test is also the topic of #9203, but I think the failure mode in this case is quite different and maybe unrelated to the specific test entirely.)

This test failed on a CI run on #10218:

https://github.com/oxidecomputer/omicron/pull/10218/checks?check_run_id=70617039381

Log showing the specific test failure:

https://buildomat.eng.oxide.computer/wg/0/details/01KNS7RJY1NSFCV2V4TFGY15SM/xhNZF9P3KK7KG4fFixQFB4tRCm0LfJVzU1QMY2yNRKbzTGeZ/01KNS7S1T5CCZ5267PJDXCJQ98

Excerpt from the log showing the failure:

8955	2026-04-09T14:02:49.024Z	    log file: /var/tmp/omicron_tmp/test_all-bca6a1d309cf2b26-test_disk_move_between_instances.186476.0.log
8956	2026-04-09T14:02:49.024Z	    note: configured to log to "/var/tmp/omicron_tmp/test_all-bca6a1d309cf2b26-test_disk_move_between_instances.186476.0.log"
8957	2026-04-09T14:02:49.024Z	    DB URL: postgresql://root@[::1]:34215/omicron?sslmode=disable
8958	2026-04-09T14:02:49.024Z	    DB address: [::1]:34215
8959	2026-04-09T14:02:49.024Z	    log file: /var/tmp/omicron_tmp/test_all-bca6a1d309cf2b26-test_disk_move_between_instances.186476.2.log
8960	2026-04-09T14:02:49.024Z	    note: configured to log to "/var/tmp/omicron_tmp/test_all-bca6a1d309cf2b26-test_disk_move_between_instances.186476.2.log"
8961	2026-04-09T14:02:49.024Z	
8962	2026-04-09T14:02:49.024Z	    thread 'integration_tests::disks::test_disk_move_between_instances' (186477) panicked at sp-sim/src/sidecar.rs:281:60:
8963	2026-04-09T14:02:49.024Z	    called `Result::unwrap()` on an `Err` value: couldn't understand ereport header from [::1]:51615: Size(SizeError)

nextest reran the test and noted that it passed the second time.

Cc @hawkw since this looks ereport-related rather than something in test_disk_move_between_instances itself.

Start with sp-sim/src/sidecar.rs:281 and the integration_tests::disks::test_disk_move_between_instances test. Run the test with nextest and compare its output with the linked CI log, focusing on the ereport header parsing failure. Done means identifying and fixing the intermittent failure so the test passes reliably.