chore: adds transactional persistent store and recovery

[tanderson-ld]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

SDK-1610

Describe the solution you've provided

Ports Dotnet impl to Java

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Note

Implements a write-through architecture and improves persistent store recovery and serialization.

• Adds WriteThroughStore to read from persistence during init, then switch to memory; writes go to both in READ_WRITE mode; supports transactional apply
• Introduces PersistentDataStoreConverter to serialize/deserialize between in-memory ItemDescriptor and persistent SerializedItemDescriptor
• Extends PersistentDataStoreWrapper to use the converter and accept an external CacheExporter (via SettableCache) to repopulate persistence after outages; retains cache-based fallback
• Updates FDv2DataSystem to compose WriteThroughStore, build optional persistent store, and set memory store as recovery exporter
• Adds comprehensive tests for converter, wrapper recovery behavior, and write-through store semantics

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