test(ecstore): add shared MockWarmBackend test utility for lifecycle and tier tests (#4716)

* test(ecstore): extract shared MockWarmBackend into a test-util feature (backlog#1148 ilm-6)

The tier/lifecycle integration tests carried two byte-for-byte copies of an
in-memory WarmBackend mock — one in crates/scanner/tests and one in
rustfs/src/app — plus duplicated register_mock_tier and polling helpers. Both
implemented the same ecstore WarmBackend trait.

Consolidate them into ecstore behind a new `test-util` feature, exposed via the
`rustfs_ecstore::api::tier::test_util` facade:

- MockWarmBackend: in-memory WarmBackend with an operation log (for ordering
  assertions such as "local delete precedes remote remove") and fault injection
  (FaultConfig): unreachable, HTTP 5xx, credential rejection, injected latency,
  plus external_remove to simulate an out-of-band remote deletion.
- register_mock_tier / register_mock_tier_backend: register the mock into any
  TierConfigMgr handle (the global manager used by scanner tests or a
  per-instance one used by the app tests).
- xl.meta transition assertion helpers: read_transition_meta,
  assert_transition_meta_consistent (cross-shard consistency of the
  status/tier/remote-key/remote-version-id tuple plus free-version count), and
  free_version_count.
- polling helpers: wait_for_remote_absence, wait_for_object_count,
  wait_for_free_version_absence.

Both existing copies now consume this single definition; `rg 'struct
MockWarmBackend'` collapses to one. The feature is enabled only from
[dev-dependencies], so it never links into the production binary (resolver 3).

Designed for downstream ilm-8 (restore lifecycle) and ilm-11 (tier fault
injection matrix). Coordinates with #4706 (ilm-2), which adds op-logging to the
scanner mock — that op-logging is now part of this shared surface, so #4706
should rebase onto it.

Refs rustfs/backlog#1148 (ilm-6), rustfs/backlog#1155.

* test(ecstore): fix shared MockWarmBackend usage after main merge

- Access stored objects via MockWarmBackend::contains() instead of the now
  private inner objects map (fixes E0609 after the shared test-util refactor).
- Drop dead ReadCloser/ReaderImpl/DiskAPI imports and the unused
  transition_api test re-exports the mock extraction left behind.
- Reword the scanner/rustfs test-util dependency comments so they no longer
  embed the literal rustfs_ecstore:: path that trips the ECStore
  architecture-migration guard.

crates/ecstore/src/api/mod.rs

@@ -429,4 +429,13 @@ pub mod tier {
             WarmBackend, WarmBackendGetOpts, WarmBackendImpl, build_transition_put_options, check_warm_backend, new_warm_backend,
         };
     }
+
+    #[cfg(feature = "test-util")]
+    pub mod test_util {
+        pub use crate::services::tier::test_util::{
+            FaultConfig, MockStoredObject, MockWarmBackend, MockWarmOp, TransitionMeta, assert_transition_meta_consistent,
+            free_version_count, read_transition_meta, register_mock_tier, register_mock_tier_backend,
+            wait_for_free_version_absence,
+        };
+    }
 }