[fix][client] Don't pin broker-assigned redirect URL across reconnect retries

[merlimat]

Fixes #25997

Motivation

#25929 added an explicitHostURI pin to ConnectionHandler so the v5 transaction coordinator's metadata-store discovery re-dials the elected leader on retry instead of falling back to the service URL. The same change also persisted the broker-assigned redirect URL from CommandCloseProducer/CommandCloseConsumer unload notifications (PIP-307) into that pin: a producer/consumer that was redirected once stayed pinned to that address on every subsequent retry, never going through topic lookup again, and nothing ever cleared the field.

A redirect to a wrong or stale broker — e.g. a stepping-down ExtensibleLoadManagerImpl leader whose closeInternalTopics() redirects internal-topic clients to itself via the stale channel-table assignment — now wedged the client permanently with ServiceUnitNotReadyException instead of recovering via lookup on the next retry. This is the cause of the ExtensibleLoadManagerImplTest.testRoleChange flakiness on master. Before #25929 the redirect was honored for the immediate reconnect attempt only, so one failed attempt was followed by a lookup that found the new owner.

Modifications

• ConnectionHandler.connectionClosed() updates the explicitHostURI pin only for handlers already in explicit-host mode, i.e. the TC metadata-store discovery, which entered it via grabCnx(URI, boolean). The pin update is still needed there: TransactionMetaStoreHandler.retargetLeader() switches leaders by closing the channel and passing the new leader URI through connectionClosed(), and a failed first dial to the new leader must not retry against the old one.
• Lookup-based handlers (producers/consumers) never enter explicit-host mode, so the broker-assigned redirect is honored for the immediate reconnect attempt only; if that attempt fails, retries fall back to topic lookup, restoring the pre-[improve][broker] PIP-473 P5.3: metadata-store TC leader election + assignment watch #25929 behavior.
• Added a regression test that delivers a disconnect+redirect with an unreachable assignedBrokerServiceUrl to a live producer and consumer (exactly as ClientCnx#handleCloseProducer/#handleCloseConsumer do) and asserts both recover via lookup and can exchange a message.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• BrokerClientIntegrationTest.testStaleBrokerRedirectFallsBackToLookup fails on master without the fix (the clients stay pinned to the unreachable address) and passes with it.
• TransactionClientConnectTest passes, covering the TC client connection path.
• ExtensibleLoadManagerImplTest.testRoleChange with invocationCount = 20 passes 40/40 invocations with the fix (the test factory runs both ServiceUnitStateChannel implementations), vs. 1-2 failures per 20 on master per [Bug][client] Regression in #25929: broker-assigned redirect URL is permanently pinned by ConnectionHandler, breaking lookup fallback on reconnect #25997.