feat: Drop persistent-store cache after FDv2 in-memory store init

[keelerm84]

Summary

With FDv2, the in-memory store (InMemoryFeatureStoreV2) becomes the source of truth for flag evaluations once it receives its first full payload. The persistent store's CachingStoreWrapper continues to hold a duplicate copy of every flag and segment in its ExpiringCache, even though that cache is never read again post-init -- roughly doubling the in-memory flag footprint when a persistent store is configured.

This change adds a disable_cache hook that propagates from Store#set_basis through FeatureStoreClientWrapperV2 (and the RedisFeatureStore facade) down to CachingStoreWrapper, where it releases the ExpiringCache reference. The cache is still populated and useful during the bootstrap window before set_basis fires, so :expiration / :capacity options remain functional during that window; YARD docs on the Redis, DynamoDB, and Consul integrations note the new FDv2 behavior.

While here, this also closes a latent TOCTOU window in CachingStoreWrapper's read paths by capturing @cache to a local variable before the existing nil-guards, so a concurrent disable_cache cannot NPE an in-flight reader.

Mirrors the Python implementation from python-server-sdk PR #426.

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Note

Medium Risk
Changes cache lifecycle behavior for persistent stores by disabling their local cache after the first full FDv2 payload; mistakes could impact persistent-store performance or initialization sequencing. Includes concurrency-sensitive cache handling and new warning-path logging that should be validated under load.

Overview
With FDv2, the SDK now drops the persistent-store in-memory cache once the in-memory store becomes authoritative (after a full TRANSFER_FULL payload), reducing duplicate flag/segment memory usage.

This introduces a disable_cache hook that propagates through Store#set_basis (best-effort with warning on failure), FeatureStoreClientWrapperV2, and the Redis feature-store facade down to Integrations::Util::CachingStoreWrapper, which releases/clears its cache and updates read paths to safely capture @cache locally to avoid races.

Updates YARD docs for Redis/DynamoDB/Consul cache options to note the FDv2 bootstrap-only behavior, and adds specs covering forwarding/no-op behavior, idempotency, race-safe bypass after disable, and tolerance of missing/raising disable_cache implementations.

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