refactor: async CacheProvider API + reduce public surface

- Make on_lookup/on_store async on CacheProvider ABC
- Simplify CacheContext: replace cache_key + cache_key_bytes with
  cache_key_hash (str hex digest)
- Make registry/utility functions internal (_prefix)
- Trim comfy_api.latest.Caching exports to core API only
- Make cache get/set async throughout caching.py hierarchy
- Use asyncio.create_task for fire-and-forget on_store
- Add NaN gating before provider calls in Core
- Add await to 5 cache call sites in execution.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

comfy_api/latest/__init__.py

@@ -118,12 +118,12 @@ class Caching:
         from comfy_api.latest import Caching
 
         class MyRedisProvider(Caching.CacheProvider):
-            def on_lookup(self, context):
+            async def on_lookup(self, context):
                 # Check Redis for cached result
                 ...
 
-            def on_store(self, context, value):
-                # Store to Redis (can be async internally)
+            async def on_store(self, context, value):
+                # Store to Redis
                 ...
 
         Caching.register_provider(MyRedisProvider())
@@ -135,10 +135,6 @@ class Caching:
         CacheValue,
         register_cache_provider as register_provider,
         unregister_cache_provider as unregister_provider,
-        get_cache_providers as get_providers,
-        has_cache_providers as has_providers,
-        clear_cache_providers as clear_providers,
-        estimate_value_size,
     )