core,api: Make BaseSnapshot and Schema lazy fields thread-safe [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Fixes #17585

RESTTableCache caches TableMetadata and shares it across threads. TableMetadata lazily-initializes fields in BaseSnapshot and Schema without synchronization, while PartitionSpec correctly uses synchronized + volatile (double-checked locking).

This PR applies the same thread-safe lazy-initialization pattern:

• BaseSnapshot: mark 7 lazy fields volatile and guard allManifests/dataManifests/deleteManifests/addedDataFiles/removedDataFiles/addedDeleteFiles/removedDeleteFiles getters with double-checked locking (synchronized (this) + inner null check).
• Schema: mark 7 lazy fields volatile and guard the 6 lazy*() initializers (lazyIdToField, lazyNameToId, lazyIdToName, lazyLowerCaseNameToId, lazyIdToAccessor, lazyIdentifierFieldIdSet) with double-checked locking.

This eliminates the data race on lazily-initialized fields when TableMetadata is shared across threads (e.g. via RESTSessionCatalog's RESTTableCache), matching the existing PartitionSpec pattern.