feat(client): add idle-management methods to the composable pool Cache

[aajtodd]

Add three methods to client::pool::Cache / Cached for managing idle cached services explicitly:

• Cached::discard(self) — consume the checkout and drop the inner service without returning it to the pool.
• Cache::try_pop_idle() — remove and return one idle service, raw (no return-to-pool wrapper).
• Cache::try_checkout_idle() — take one idle service wrapped so it returns to the pool on drop, without making a new one when none is idle.

Motivation

Cache currently exposes call (take a service, making one if the pool is empty) and retain (bulk eviction by predicate). Two operations a pool consumer needs have no API:

1. Drop a checked-out service that has become unusable. The only way to skip reinsertion on drop is for the inner service's poll_ready to return Err. A consumer that learns a service is bad after checkout — for a connection, the peer sent Connection: close, an HTTP/2 GOAWAY arrived on the response, or an error classifier fired — must either force a synthetic poll_ready error or let the known-bad service return to the pool. discard makes the intent direct and leaves poll_ready for readiness alone.

2. Reach an idle service without making a new one. call always produces a service, starting a new one when the pool is empty. A consumer that wants to act on existing idle capacity only — release it to free the resource it holds, or hand it out for a single use — cannot express "idle if present, otherwise nothing." try_pop_idle (release) and try_checkout_idle (single use, returns on drop) cover the two shapes.

Solution

The three methods are thin wrappers over the existing shared service list, serialized by the same lock as call and retain:

Method	Behavior
Cached::discard(self)	Sets the is_closed flag the Drop impl already checks, so the inner service is dropped instead of reinserted.
Cache::try_pop_idle()	Pops one entry from the idle list and returns it raw.
Cache::try_checkout_idle()	Pops one entry and wraps it in Cached, matching what call returns for an idle hit. Returns None rather than connecting when the pool is empty.

No existing behavior changes.

Tests

Added to the cache test module, using the existing tower_test::mock harness:

• discard prevents reinsertion (cache empty after a discarded checkout).
• try_pop_idle removes one idle service and does not reinsert it.
• try_checkout_idle returns the service to the pool on drop and makes no new service (a single connection is reused).
• both takes return None on an empty cache.

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See also smithy-lang/smithy-rs#4708