Support cache stores without delete_matched

[rience]

Summary

Rack::Attack.reset! no longer raises IncompatibleStoreError for cache stores that don't support delete_matched (e.g., SolidCache). Instead, it falls
back to a prefix rotation strategy that orphans old keys. Stores that fully support delete_matched are completely unaffected.

Motivation

SolidCache inherits from ActiveSupport::Cache::Store which defines delete_matched but raises NotImplementedError. This means reset! fails even though
all runtime operations (read, write, increment, delete) work fine. Since reset! is primarily a test helper for clearing state between runs, it
shouldn't make an otherwise functional store "incompatible".

How it works

• reset! attempts to call delete_matched as before
• If the store raises NotImplementedError (e.g., SolidCache inheriting ActiveSupport's stub) or NoMethodError (store doesn't define it at all), it falls
  back to prefix rotation
• A generation counter is stored in the cache under "rack::attack:generation" and incremented on each fallback reset!
• All key operations use an effective_prefix that includes the generation (e.g., rack::attack:g1:...) when generation > 0, making old keys inaccessible
• The generation is read from the cache once on first access and memoized in memory — nearly zero runtime cost
• Old keys are not deleted but expire naturally via TTL
• IncompatibleStoreError remains defined but is no longer raised, avoiding breakage for code that rescues it