feat(cache-provider): add on_set_prompt lifecycle hook for providers

Adds a new on_set_prompt() lifecycle hook on CacheProvider that fires
after the cache key set is prepared for a new prompt. Dispatched via
asyncio.create_task with errors swallowed (same fail-safe pattern as
on_store / on_lookup).

Why: BasicCache's lifecycle notifications to external providers were
incomplete. set_prompt is a key per-prompt event that providers need
visibility into — for example, to reset per-prompt timing/state used
for cost-aware caching policies (a provider can set t=0 here, then
measure elapsed at each on_store to estimate compute saved by a hit).

Backward-compatible: default implementation is a no-op, existing
providers compile and run unchanged. Providers that need the per-prompt
boundary override on_set_prompt().

comfy_api/latest/_caching.py

@@ -21,6 +21,10 @@ class CacheProvider(ABC):
     Exceptions from provider methods are caught by the caller and never break execution.
     """
 
+    async def on_set_prompt(self) -> None:
+        """Called after prompt cache keys are prepared. Dispatched via asyncio.create_task."""
+        pass
+
     @abstractmethod
     async def on_lookup(self, context: CacheContext) -> Optional[CacheValue]:
         """Called on local cache miss. Return CacheValue if found, None otherwise."""

comfy_execution/caching.py

@@ -164,6 +164,7 @@ class BasicCache:
         await self.cache_key_set.add_keys(node_ids)
         self.is_changed_cache = is_changed_cache
         self.initialized = True
+        await self._notify_providers_set_prompt()
 
     def all_node_ids(self):
         assert self.initialized
@@ -263,6 +264,24 @@ class BasicCache:
             except Exception as e:
                 _logger.warning(f"Cache provider {provider.__class__.__name__} error on store: {e}")
 
+    async def _notify_providers_set_prompt(self):
+        from comfy_execution.cache_provider import (
+            _has_cache_providers, _get_cache_providers, _logger
+        )
+
+        if not self.enable_providers:
+            return
+        if not _has_cache_providers():
+            return
+
+        for provider in _get_cache_providers():
+            try:
+                task = asyncio.create_task(self._safe_provider_set_prompt(provider))
+                self._pending_store_tasks.add(task)
+                task.add_done_callback(self._pending_store_tasks.discard)
+            except Exception as e:
+                _logger.warning(f"Cache provider {provider.__class__.__name__} error on set_prompt: {e}")
+
     @staticmethod
     async def _safe_provider_store(provider, context, cache_value):
         from comfy_execution.cache_provider import _logger
@@ -271,6 +290,14 @@ class BasicCache:
         except Exception as e:
             _logger.warning(f"Cache provider {provider.__class__.__name__} async store error: {e}")
 
+    @staticmethod
+    async def _safe_provider_set_prompt(provider):
+        from comfy_execution.cache_provider import _logger
+        try:
+            await provider.on_set_prompt()
+        except Exception as e:
+            _logger.warning(f"Cache provider {provider.__class__.__name__} async set_prompt error: {e}")
+
     async def _check_providers_lookup(self, node_id, cache_key):
         from comfy_execution.cache_provider import (
             _has_cache_providers, _get_cache_providers,