4.x: Unify throttler permit release across request handlers #1228

Problem

Every request handler returns its throttler permit "exactly once", but each one does it differently:

• CqlRequestHandler: an admitted flag, a registrationComplete flag, a throttlerReleased CAS, and a saved terminalError
• GraphRequestHandler and CqlPrepareHandler: whichever completeExceptionally wins, a CancellationException carve-out, and the exceptionally hook
• ThrottledAdminRequestHandler: the admitted flag only
• ContinuousRequestHandlerBase: the return value of abortGlobalRequestOrChosenCallback and the cancel listener

After #1007 the handlers already behave differently. Only CqlRequestHandler defers a release when a timeout races register(), and only it returns early from onThrottleReady() when the result is already complete. GraphRequestHandler and CqlPrepareHandler still record THROTTLING_DELAY and build a query plan for a request that timed out or was cancelled while its ready callback was pending. That is harmless today only because sendRequest() checks result.isDone().

Every review round of #1007 found the same defect again in a sibling handler.

Expected behavior

One small shared helper handles the permit for every handler: it marks the request admitted, releases the permit once, and chooses signalTimeout, signalCancel or signalError from the result's state. Every handler uses it, so #949, #1013 and #1014 build on one tested mechanism.

Related

• fix: contain throttled request setup failures #1007
• 4.x: onThrottleReady exceptions can escape or leave request futures incomplete #949, 4.x: Continuous requests can leak throttler permits on terminal pre-admission paths #1013, 4.x: CqlPrepareHandler leaks lifecycle resources on synchronous setup failure #1014