Refactor index cache to store data-only IndexData and rehydrate indices without IO

Problem

The index cache currently stores full index structs that hold object store readers/connections. This is risky in multitenant scenarios and when credentials expire. It also makes cache sizing unreliable because cached entries share Arc state outside the cache.

Goal

Change the index cache to store data-only IndexData and rehydrate index instances using fresh object store connections. Rehydration must use minimal CPU and no IO.

Approach

1. Introduce internal IndexData structs per index type. They must be data-only (no readers, stores, schedulers).
2. Cache IndexData in the existing index cache (new keys like ScalarIndexDataKey, VectorIndexDataKey).
3. Replace caching of Arc<dyn Index> with cached IndexData and per-call rehydration.
4. Keep sub-index partition/page caches, but ensure their entries are also data-only.

Enforcement

• Create tests that assert rehydration performs zero object store IO. Use io_stats() for this.
• Create tests that assert memory size checks are exactly correct: https://github.com/lance-format/lance/pull/5152/changes#diff-664229ef83b65cc75050853551318f025542f7a2f03884c468b1f3f43e4cc37d

———

Example: BTree

  Index Cache (data-only)                 Partition/Page Cache (data-only)
  ┌────────────────────────┐              ┌──────────────────────────────┐
  │ BTreeIndexData         │              │ BTreePageKey -> FlatIndex    │
  │ - page_lookup          │              │ (page content only)          │
  │ - ranges_to_files      │              └──────────────────────────────┘
  │ - data_type, batch_size│
  └────────────────────────┘
            │
            │ from_data (no IO)
            ▼
  ┌────────────────────────┐
  │ BTreeIndex (runtime)   │
  │ - store (fresh)        │
  │ - index_cache (weak)   │
  │ - lazy readers         │
  └────────────────────────┘
            │
            │ on page request (IO)
            ▼
        read page -> build FlatIndex -> cache FlatIndex data only

Example: IVF_PQ

  Index Cache (data-only)                 Partition Cache (data-only)
  ┌────────────────────────┐              ┌────────────────────────────────────┐
  │ IvfIndexData           │              │ IVFPartitionKey -> PartitionEntry  │
  │ - ivf_model            │              │ - subindex data (e.g. Flat/HNSW)   │
  │ - sub_index_metadata   │              │ - partition storage data           │
  │ - distance_type        │              └────────────────────────────────────┘
  │ - pq codebook/metadata │
  └────────────────────────┘
            │
            │ from_data (no IO)
            ▼
  ┌────────────────────────┐
  │ IVFIndex (runtime)     │
  │ - lazy index reader    │
  │ - lazy storage reader  │
  │ - index_cache (weak)   │
  └────────────────────────┘
            │
            │ on partition load (IO)
            ▼
    read partition -> build subindex -> cache PartitionEntry data only