4.x: Continuous requests can leak throttler permits on terminal pre-admission paths

[dkropachev]

Problem

ContinuousRequestHandlerBase does not consistently notify the throttler when a request becomes terminal without selecting a node callback.

Two affected paths:

• An admitted request that exhausts its query plan completes with AllNodesFailedException but never calls signalError, permanently consuming a concurrency permit.
• A queued continuous Graph request can hit its global timeout before admission. It is not removed from the throttler; if admitted later, it performs no write and permanently consumes the transferred permit.

Expected behavior

Track queued versus admitted ownership and return or remove the permit exactly once on every terminal path.

Related

• 4.x: CQL throttler slot and request timeout leak when onThrottleReady() throws #980 tracks synchronous request-setup cleanup; these admission-state failures are intentionally deferred from it.