fix(batch): release pooled model slots on failures and cancellation

[YMuskrat]

Fixes #760.

PooledChatModel reserves a slot before constructing ChatOpenAI, but construction currently happens outside cleanup. Repeated construction errors can exhaust the pool even though no request is running. Cancelled async calls also skip cleanup, and cancellation while waiting on a threaded blocking acquire can leave an orphaned waiter that reserves capacity later.

Put client construction and invocation inside a try/finally release in both retry loops. For async capacity waiting, retry the existing non-blocking acquire after a cancellable 50 ms sleep, avoiding a background acquiring thread. This adds up to one polling interval of capacity-wakeup latency when the pool is full. Existing rate-limit backoff, retries and callback forwarding are retained.

This is per-call cleanup, complementary to the owner/process cleanup proposed in #743.

Validation: make lint, make format, and make format-check pass; all 9,965 unit tests and 124 integration tests pass. Seven new cases cover construction failures, in-flight cancellation, waiting cancellation, capacity recovery and sync/async rate-limit retries. Four fail on the original implementation. The focused run including the existing contrib pool tests passes all 36 tests; no live provider calls were made.