feat: hash API keys used internally for caching

[joshmyers-medibank]

Summary

• Hash sdkKey-derived persistence identifiers for cached datafiles, persisted Last-Modified metadata, and ODP event queue storage.
• Update storage test helpers and add coverage for hashed persistence names, legacy raw-key cache misses, and hashed ODP queue persistence.
• Fix the datafile download session cleanup ordering so the affected tests complete without leaking mock sessions.

This aligns the Swift SDK with issue #634 by avoiding raw API key persistence in on-device cache names and UserDefaults keys, while still using the raw key where it is required for network requests. Legacy raw-named cache entries are intentionally not migrated, so existing persisted cache state is treated as a one-time cache miss after upgrade.

Test plan

• xcodebuild test -quiet -workspace OptimizelySwiftSDK.xcworkspace -scheme OptimizelySwiftSDK-iOS -destination 'platform=iOS Simulator,name=iPhone 16,OS=latest' -only-testing:OptimizelyTests-Common-iOS/DatafileHandlerTests -only-testing:OptimizelyTests-MultiClients-iOS/DatafileHandlerTests_MultiClients -only-testing:OptimizelyTests-APIs-iOS/OptimizelyClientTests_DatafileHandler -only-testing:OptimizelyTests-Common-iOS/OdpEventManagerTests
• xcodebuild test -quiet -workspace OptimizelySwiftSDK.xcworkspace -scheme OptimizelySwiftSDK-tvOS -destination 'platform=tvOS Simulator,name=Apple TV,OS=latest' -only-testing:OptimizelyTests-Common-tvOS/DatafileHandlerTests -only-testing:OptimizelyTests-Common-tvOS/OdpEventManagerTests
• Verified the new coverage for hashed datafile cache naming, hashed Last-Modified storage, legacy raw-key cache behavior, and hashed ODP event queue persistence.

Issues

• [ENHANCEMENT] Hash API keys used internally for caching #634