test failed in CI: app::sagas::disk_create::test::test_action_failure_can_unwind_idempotently

This test failed on a CI run on PR #10108: https://github.com/oxidecomputer/omicron/pull/10108/checks?check_run_id=67947973228

Log showing the specific test failure: https://buildomat.eng.oxide.computer/wg/0/details/01KM66YK4546W0P4JC0PTHC9TR/GG4HutHIxijs894HtA3ss6MHSGAMt3n5g9QTRHac275Gkh0o/01KM66ZTEBB84REWSXZJ2AW9QN#S7048

Excerpt from the log showing the failure:

7069	2026-03-20T18:35:41.862Z	    thread 'app::sagas::disk_create::test::test_action_failure_can_unwind_idempotently' (2) panicked at nexus/src/app/sagas/disk_create.rs:1382:9:
7070	2026-03-20T18:35:41.862Z	    assertion failed: no_disk_records_exist(datastore).await
7071	2026-03-20T18:35:41.862Z	    stack backtrace:
7072	2026-03-20T18:35:41.862Z	       0: __rustc::rust_begin_unwind
7073	2026-03-20T18:35:41.862Z	                 at /rustc/4a4ef493e3a1488c6e321570238084b38948f6db/library/std/src/panicking.rs:689:5
7074	2026-03-20T18:35:41.863Z	       1: core::panicking::panic_fmt
7075	2026-03-20T18:35:41.863Z	                 at /rustc/4a4ef493e3a1488c6e321570238084b38948f6db/library/core/src/panicking.rs:80:14
7076	2026-03-20T18:35:41.863Z	       2: core::panicking::panic
7077	2026-03-20T18:35:41.863Z	                 at /rustc/4a4ef493e3a1488c6e321570238084b38948f6db/library/core/src/panicking.rs:150:5
7078	2026-03-20T18:35:41.863Z	       3: {async_fn#0}
7079	2026-03-20T18:35:41.863Z	                 at ./src/app/sagas/disk_create.rs:1382:9
7080	2026-03-20T18:35:41.863Z	       4: {async_block#0}
7081	2026-03-20T18:35:41.863Z	                 at ./src/app/sagas/disk_create.rs:1473:71
7082	2026-03-20T18:35:41.863Z	       5: poll<alloc::boxed::Box<(dyn core::future::future::Future<Output=()> + core::marker::Send), alloc::alloc::Global>>
7083	2026-03-20T18:35:41.863Z	                 at /rustc/4a4ef493e3a1488c6e321570238084b38948f6db/library/core/src/future/future.rs:133:9
7084	2026-03-20T18:35:41.863Z	       6: {async_fn#0}<omicron_nexus::app::sagas::disk_create::SagaDiskCreate, omicron_nexus::app::sagas::disk_create::test::test_action_failure_can_unwind_idempotently::{async_block#0}::test_action_failure_can_unwind_idempotently::{async_fn#0}::{closure_env#0}, omicron_nexus::app::sagas::disk_create::test::test_action_failure_can_unwind_idempotently::{async_block#0}::test_action_failure_can_unwind_idempotently::{async_fn#0}::{closure_env#1}>
7085	2026-03-20T18:35:41.863Z	                 at ./src/app/sagas/test_helpers.rs:754:22

Start with nexus/src/app/sagas/disk_create.rs, especially test_action_failure_can_unwind_idempotently and the assertion at line 1382. Review the CI log and the helper path in src/app/sagas/test_helpers.rs:754 to determine why cleanup leaves disk records behind. Done means the test passes and no disk records remain after the failed action unwinds.