fix(ecstore): never cache fabricated bucket metadata as authoritative (#5307)

BucketMetadataSys::get_config lazily fabricated a default BucketMetadata
(object-lock off) for any bucket whose .metadata.bin was ConfigNotFound and
cached it in the map that the map-only, fail-closed metadata_sys::get()
serves. The object-lock batch-delete gate (object_lock_delete_check_required,
backlog#929 / #4297) treats that map as authoritative, so a metadata miss
became a cached "no lock" answer: a versioning peek could poison the cache
and let delete_objects skip the per-object retention/legal-hold stat. The
same fabrication raced make_bucket (lost update overwriting freshly
persisted lock-enabled metadata) and let the 15-minute refresh loop replace
good cached metadata on a transient quorum dip.

Production changes:
- get_config caches only metadata actually read from disk; misses are
  recorded in a bounded negative cache (30s TTL, 10k entries, invalidated by
  set()) so repeated lookups for metadata-less names cost no extra
  namespace-lock + erasure-set fanout (reachable pre-auth via CORS
  preflight and per-key in DeleteObjects).
- concurrent_load never lets a fabricated default REPLACE an existing map
  entry; startup insert-if-vacant behavior for legacy buckets is preserved.
- delete_objects and new_ns_lock resolve dist-erasure, versioning, and the
  object-lock gate from the set's own instance context (backlog#1052)
  instead of the ambient facade, so a second in-process instance (or, in
  tests, another test's transient DistErasure window) cannot reroute
  locking onto an empty dist locker list or answer with the wrong
  instance's bucket state.

Test-isolation changes (the bug that surfaced all of the above: the
delete_objects lock-gating test failed deterministically when sharing a
process with the lifecycle env tests):
- The MinIO-migration test builds on an isolated InstanceContext instead of
  registering soon-deleted disks in the shared bootstrap registry.
- The cached lifecycle env re-registers its disks on every use, surviving
  other serial tests' reset_local_disk_test_state.
- Hermetic SetDisks helpers gain isolated-context variants pinned to plain
  erasure; tier-free non-serial test modules use them, guard-based
  SetupTypeGuard tests stay on the bootstrap context.
- Three deterministic pin tests (nextest-safe) cover the caching contract,
  the delete gate resolution source, and the ns-lock resolution source.

Verification:
- cargo test -p rustfs-ecstore --lib -- --exact <4-test combo from the
  report> (previously failing, now green)
- cargo test -p rustfs-ecstore --lib: 3169 passed / 0 failed across
  repeated runs; cargo fmt --check and cargo clippy --lib --tests clean
- Adversarial validation (high-risk tier, all seven roles) run per
  AGENTS.md; all findings fixed or rebutted with evidence

crates/ecstore/src/bucket/metadata.rs

@@ -649,7 +649,7 @@ impl BucketMetadata {
         Ok(())
     }
 
-    fn default_timestamps(&mut self) {
+    pub(crate) fn default_timestamps(&mut self) {
         if self.policy_config_updated_at == OffsetDateTime::UNIX_EPOCH {
             self.policy_config_updated_at = self.created
         }
@@ -1093,16 +1093,25 @@ pub async fn load_bucket_metadata(api: Arc<ECStore>, bucket: &str) -> Result<Buc
 }
 
 pub async fn load_bucket_metadata_parse(api: Arc<ECStore>, bucket: &str, parse: bool) -> Result<BucketMetadata> {
-    let mut bm = match read_bucket_metadata(api.clone(), bucket).await {
-        Ok(res) => res,
+    Ok(load_bucket_metadata_parse_with_presence(api, bucket, parse).await?.0)
+}
+
+/// The returned `bool` reports whether the metadata was actually read from
+/// persisted storage; `false` means no metadata exists for this bucket on this
+/// store and the returned value is a fabricated in-memory default.
+pub(crate) async fn load_bucket_metadata_parse_with_presence(
+    api: Arc<ECStore>,
+    bucket: &str,
+    parse: bool,
+) -> Result<(BucketMetadata, bool)> {
+    let (mut bm, persisted) = match read_bucket_metadata(api.clone(), bucket).await {
+        Ok(res) => (res, true),
         Err(err) => {
             if err != Error::ConfigNotFound {
                 return Err(err);
             }
 
-            // info!("bucketmeta {} not found with err {:?}, start to init ", bucket, &err);
-
-            BucketMetadata::new(bucket)
+            (BucketMetadata::new(bucket), false)
         }
     };
 
@@ -1112,7 +1121,7 @@ pub async fn load_bucket_metadata_parse(api: Arc<ECStore>, bucket: &str, parse:
         bm.parse_all_configs()?;
     }
 
-    Ok(bm)
+    Ok((bm, persisted))
 }
 
 async fn read_bucket_metadata(api: Arc<ECStore>, bucket: &str) -> Result<BucketMetadata> {