Improve Delivery caching reliability and invalidation

This PR fixes several issues found during a review of the Delivery caching layer and simplifies some of the related internals.

What changed

Fail-safe behavior

Fail-safe now distinguishes between an unavailable origin and a valid response indicating that no value exists.

• No response or retryable errors can serve stale content through fail-safe.
• Non-retryable responses such as 404 return normally and remove the stale cached value.
• IDeliveryCacheManager keeps the same API; the behavior of null vs. an exception is now documented.

Cache invalidation

• Fixed invalidations expiring after FusionCache's default 30-second duration.
• Tag invalidations now use TagsDefaultEntryOptions, which defaults to 10 days and can be configured by consumers.
• Invalidation keys are normalized and matched case-insensitively.
• Added DeliveryCacheDependencies.ForItem, ForType, ForTaxonomy, and ForAsset for composing dependency keys.

Cache isolation and Redis

• FusionCache now owns the cache key prefix through CacheKeyPrefix.
• Distributed keys are versioned as {KeyPrefix}:{EnvironmentId}:v1:{key}.
• Purging one client no longer clears other clients sharing the same cache.
• Redis failures use a short circuit breaker instead of failing cached queries.
• FusionCache logging is now wired through the standard logging setup.
• Cache entries declare a size of 1, so size-limited memory caches work correctly.

Existing Redis entries using the previous key format will remain unused until they expire.

Configuration and cache manager access

• ConfigureFusionCache settings now apply to SDK cache operations.
• Writes start from the consumer's DefaultEntryOptions and override only SDK-specific settings.
• The default cache manager can now be resolved without a keyed service.
• DeliveryClient.CacheManager exposes the cache manager for standalone clients.

Internal cleanup

• Added CacheResult<T>.IsStale for per-request fail-safe state.
• Removed the previous event-handler/state-provider approach.
• Merged MemoryCacheManager and HybridCacheManager into FusionCacheManager.
• Item and listing queries now share the same cached fetch implementation.

Public API changes

Added:

• DeliveryClient.CacheManager
• CacheResult<T>.IsStale
• DeliveryCacheDependencies.ForItem
• DeliveryCacheDependencies.ForType
• DeliveryCacheDependencies.ForTaxonomy
• DeliveryCacheDependencies.ForAsset

No public APIs were removed.

Consumer-visible behavior

• A 404 or other non-retryable response after invalidation no longer serves stale content.
• A 5xx or unavailable origin can still serve stale content when fail-safe is enabled.
• Redis outages no longer fail cached queries; requests may temporarily fall back to the origin and produce warning logs.
• ConfigureFusionCache settings such as DefaultEntryOptions now affect SDK operations.
• Redis cache keys use the new versioned format.

Verification

• Full solution builds with zero warnings.
• Test suite passes in both reference modes.
• Delivery tests increased from 1042 to 1051.
• Public API snapshots were reviewed for the new additions.

The four Redis-gated integration tests ran in CI against the pushed changes and pass, which matters here because this PR changes Redis key handling and purge behavior.

Documentation was also updated to reflect the new fail-safe behavior, invalidation rules, cache configuration, and multi-client behavior.