feature: expire stale object-store generations from Session caches (TTL/TTI)

[u70b3]

Follow-up to #7904 and #7944.

#7944 isolates store-bound vector cache entries (CachedIndexReadersKey, LegacyVectorIndexCacheKey) by object-store identity, so credential rotation produces a cache miss and a rebuild against the current store. However, entries from previous store generations are never proactively invalidated:

• Every rotation inserts a new generation of identity-keyed entries while the old generation stays resident until moka's capacity/weigher eviction.
• Old entries keep their Arc<ObjectStore> alive (via ScanScheduler holding a strong ref), so expired credential material (e.g. STS session tokens) remains reachable in process memory for the same duration.
• MokaCacheBackend configures no time_to_live/time_to_idle (rust/lance-core/src/cache/moka.rs — only max_capacity + weigher), matching the "no self-healing for 12+ hours" observation in Vector index cache in Session (index_cache/metadata_cache) never invalidates on ObjectStore/credential change, causing indefinite use of stale/expired S3 credentials #7904: after rotation the stale entries are never used again, but they also never age out.

Proposed direction

• Add time-based expiration (TTL or time-to-idle) for the store-bound, identity-keyed entry types, or a generation-aware invalidation hook when a new object-store identity is first seen for a given index UUID.
• Scope carefully: IvfIndexStateCacheKey, PartitionEntry, and LegacyIVFPartition entries are store-free and intentionally shared across generations — they must not be expired by this mechanism.

Impact today is bounded (moka capacity + weigher accounting), so this is memory/credential-hygiene rather than a correctness issue.

[u70b3]

Implemented in follow-up PR #7962, based on the latest #7944 portable-state approach.

Rather than adding TTL/TTI or a process-local object-store identity, the combined design removes the known store-bound entries from Session caches: #7944 removes/reconstructs the vector paths, and #7962 prevents live scalar indices backed by LanceIndexStore from entering the shared cache. Portable IVF/scalar state and decoded page/posting data remain reusable.

This makes stale-generation cleanup deterministic: there is no cached live reader/index left to pin an expired object store.