test(cache): end-to-end regressions for the body-cache hook P0s (#4675)

fix(ecstore): make body-cache hook re-registrable + add e2e regressions

ODC-21 (backlog#1126): the GET body-cache hook lived in a first-wins
OnceLock. When AppContext is rebuilt (config reload, test re-init) a fresh
ObjectDataCacheAdapter is constructed and re-registered, but the OnceLock
kept ecstore's GET probe pointed at adapter #1 while every usecase-layer
fill and invalidation targeted adapter #2 — silently degrading the feature
to a 0% hit rate with no error, log, or metric, and stranding entries in the
unreachable cache until their TTL.

Replace the slot with RwLock<Option<Arc<dyn GetObjectBodyCacheHook>>> so
re-registration atomically swaps to the newest adapter, and log at WARN when
a swap replaces a *different* instance (Arc::ptr_eq). RwLock over
ArcSwapOption because arc-swap's RefCnt is impl<T> (Sized, thin *mut T) and
cannot hold an Arc<dyn Trait> without a sized newtype wrapper; the probe
reads the slot once per full-object GET but only clones an Arc, negligible
next to the metadata quorum fan-out already done before the probe. Add a
test-only clear_get_object_body_cache_hook so tests register/unregister
deterministically.

With the hook now re-registrable, add true end-to-end regressions that drive
get_object_reader (not the full_object_plaintext_len predicate) against a
real erasure-coded, genuinely-compressed object via the blackbox
make_local_set_disks harness, with a stand-in hook playing the app-layer
cache (the injection point production uses; the adapter itself lives above
ecstore). These close the gap the predicate-only tests left — a caller that
opens a new shortcut serving the cached body directly, the original form of
both P0s:

- backlog#1108: a raw_data_movement_read must yield the STORED (compressed)
  bytes, never the cached plaintext.
- backlog#1109: a compressed cache hit must publish the DECOMPRESSED length
  as object_info.size (the UploadPartCopy invariant), with the streamed
  length matching.
- backlog#1146: a restore read (restore_request.days) must serve STORED
  bytes, not the cache.

Mutation-verified each e2e test bites: dropping the raw_data_movement_read
gate serves plaintext (fails #1108); removing the hit-site size republication
publishes 2972 vs 660000 (fails #1109); dropping the restore gate serves
plaintext (fails #1146).

Co-authored-by: heihutu <heihutu@gmail.com>

crates/obs/src/telemetry/dial9/disabled.rs

@@ -38,6 +38,13 @@ impl Dial9SessionGuard {
     }
 }
 
+/// Mirrors the `Drop` the enabled guard uses to flush and seal the trace
+/// segment, so a caller can drop the guard before `process::exit` under either
+/// feature without `clippy::drop_non_drop` firing on this variant.
+impl Drop for Dial9SessionGuard {
+    fn drop(&mut self) {}
+}
+
 /// Always fails: telemetry support is not compiled in.
 ///
 /// # Errors