Add `test_per_ctx_key_resource_lifecycle` to verify that per-key user
tracking correctly tears down resources independently - exercises the
fix from 02b2ef18 where a global `_Cache.users` counter caused stale
cache hits when the same `acm_func` was called with different kwargs.
Also, add a paired `acm_with_resource()` helper `@acm` that yields its
`resource_id` for per-key testing in the above suite.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Prompt-IO: ai/prompt-io/claude/20260406T172848Z_02b2ef1_prompt_io.md
