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/migration.rs

@@ -538,10 +538,12 @@ mod tests {
         use crate::disk::{BUCKET_META_PREFIX, MIGRATING_META_BUCKET, RUSTFS_META_BUCKET};
         use crate::layout::endpoints::{EndpointServerPools, Endpoints, PoolEndpoints};
         use crate::object_api::{ObjectOptions, PutObjReader};
+        use crate::runtime::instance::InstanceContext;
         use crate::storage_api_contracts::bucket::{BucketOperations, BucketOptions, MakeBucketOptions};
         use crate::storage_api_contracts::object::{ObjectIO, ObjectOperations};
-        use crate::store::{ECStore, init_local_disks};
+        use crate::store::{ECStore, init_local_disks_with_instance_ctx};
         use rustfs_utils::path::SLASH_SEPARATOR;
+        use std::sync::Arc;
         use tokio::fs;
         use tokio_util::sync::CancellationToken;
         use uuid::Uuid;
@@ -570,10 +572,20 @@ mod tests {
             cmd_line: "minio-migrate-test".to_string(),
             platform: format!("OS: {} | Arch: {}", std::env::consts::OS, std::env::consts::ARCH),
         }]);
-        init_local_disks(endpoint_pools.clone()).await.unwrap();
-        let ecstore = ECStore::new("127.0.0.1:0".parse().unwrap(), endpoint_pools, CancellationToken::new())
+        // Isolated instance context: this test deletes its disks at the end,
+        // and dead entries in the shared registry break other cached envs.
+        let instance_ctx = Arc::new(InstanceContext::new());
+        init_local_disks_with_instance_ctx(&instance_ctx, endpoint_pools.clone())
             .await
             .unwrap();
+        let ecstore = ECStore::new_with_instance_ctx(
+            "127.0.0.1:0".parse().unwrap(),
+            endpoint_pools,
+            CancellationToken::new(),
+            instance_ctx,
+        )
+        .await
+        .unwrap();
         let existing: Vec<String> = ecstore
             .list_bucket(&BucketOptions {
                 no_metadata: true,