PooledChatModel leaks API-key slots on client construction failures and cancellation #760

On main at b6f0304, contrib/batch_scan/api_pool.py reserves a pool slot before building ChatOpenAI, but construction is outside the cleanup try/except. A client-construction exception leaves the slot occupied. Repeating this can exhaust the pool and leave subsequent batch calls waiting for capacity that will never return.

A local reproduction needs no live request or real key:

import os
from contrib.batch_scan.api_pool import ApiKey, ApiKeyPool, PooledChatModel

os.environ["SSL_CERT_FILE"] = "/missing/ca-bundle.pem"
pool = ApiKeyPool([ApiKey(key="test", base_url=None, model="test", max_concurrent=1)])
model = PooledChatModel(pool)
try:
    model.invoke("test")
except Exception as exc:
    print(type(exc).__name__)
print(pool.active_requests)  # actual: 1; expected: 0
pool.acquire(timeout=0.05)  # times out despite no request being in flight

Async cancellation has two related cleanup gaps: CancelledError bypasses except Exception during an in-flight call, and a blocking acquire delegated to a thread can acquire a slot after its awaiting coroutine is cancelled.

I can wrap client construction and invocation in a finally-based release, and make async capacity waiting cancellable without an orphaned acquiring thread. Tests can cover construction failures, in-flight cancellation, waiting cancellation, normal capacity recovery and existing rate-limit retry behavior. This concerns per-call cleanup; #743 addresses owner/process lifetime cleanup separately.