Fix purgeRealms in LocalPolarisMetaStoreManagerFactory does not clear entityCacheMap

### Describe the bug `LocalPolarisMetaStoreManagerFactory.purgeRealms()` removes entries from `sessionSupplierMap` and `metaStoreManagerMap` but does not remove the corresponding entry from `entityCacheMap`. https://github.com/apache/polaris/blob/638247647c27667697675df01d2163043121e656/polaris-core/src/main/java/org/apache/polaris/core/persistence/LocalPolarisMetaStoreManagerFactory.java#L136-L139 This means after a realm is purged and re-bootstrapped, `getOrCreateEntityCache()` returns the stale cache from the previous realm lifecycle — containing references to entities that no longer exist. Additionally, `purgeRealms` is not `synchronized`, unlike the other map-mutating methods (`bootstrapRealms`, `getOrCreateMetaStoreManager`, `getOrCreateSession`), creating a potential race with concurrent reads/writes to the shared maps. ### To Reproduce _No response_ ### Actual Behavior Returns stale entires post rebootstrap ### Expected Behavior should return fresh entries after bootstrap ### Additional context _No response_ ### System information _No response_