feat(cache-provider): add on_set_prompt lifecycle hook (CORE-246)

[deepme987]

Summary

Adds a new on_set_prompt() lifecycle hook on the CacheProvider interface that fires after the cache key set is prepared for a new prompt. Dispatched as an async task with errors swallowed (same fail-safe pattern as on_store / on_lookup / on_prompt_start / on_prompt_end).

Why

BasicCache's lifecycle notifications to external providers are incomplete. set_prompt is a key per-prompt event — the moment the cache key set is prepared and the cache is ready to serve lookups — and providers currently get no signal for it. There's a clean completeness argument for surfacing this.

The immediate motivating use case: cost-aware caching policies. A provider can set t = perf_counter() in on_set_prompt, then measure elapsed at each on_store to estimate the compute that a future hit on this entry would save. That gives the provider a simple, accurate per-prompt clock without needing per-node timing exposed in the API.

Implementation

comfy_api/latest/_caching.py
  CacheProvider gains async def on_set_prompt() with default no-op

comfy_execution/caching.py
  BasicCache.set_prompt(...) now awaits self._notify_providers_set_prompt()
  _notify_providers_set_prompt → asyncio.create_task per provider
  _safe_provider_set_prompt → swallows exceptions, same as _safe_provider_store

Only 31 lines, two files, fully additive.

Backward compatibility

• New on_set_prompt() method has a pass default — existing providers compile and run unchanged.
• New _notify_providers_set_prompt is gated on self.enable_providers + _has_cache_providers(), so no-op when no providers are registered.
• Errors swallowed per existing pattern; provider faults never break execution.

Lifecycle position

on_prompt_start(prompt_id)            — already exists, fires at exec start
   ↓
caches.set_prompt(...) initializes cache_key_set
   ↓
on_set_prompt()                       — NEW, fires here
   ↓
execution loop → on_lookup / on_store per node
   ↓
on_prompt_end(prompt_id)              — already exists, fires at exec end

Test plan

• Existing CacheProvider tests still pass (on_set_prompt defaults to no-op preserves behavior)
• Manual: register a provider that overrides on_set_prompt and confirm it fires once per prompt, after cache keys are prepared

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR introduces a new provider lifecycle hook to the cache system. A new on_set_prompt() async method is added to the CacheProvider interface, allowing providers to react when a prompt is set. When BasicCache.set_prompt() is called, it now schedules provider notifications as background asyncio tasks after cache key initialization. A static helper method safely awaits each provider's hook and logs warnings if the provider raises an exception. Pending tasks are tracked in the cache for cleanup.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, covering motivation, implementation details, backward compatibility, and lifecycle positioning.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a new lifecycle hook method to the CacheProvider interface.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}