Harden failed-IP cooldown config and add DEFAULT-mode test

[hyperxpro]

Motivation:

#2221 extended failed-IP cooldown support to all LoadBalance modes and made it configurable, but left three gaps. The end-to-end feedback loop in DEFAULT mode, from markFailed to IP reordering, was never exercised through the client, leaving regressions undetected. The configuration also accepted negative cooldown periods, silently disabling the feature, and enabling the cooldown with a null period in a custom config could cause NettyRequestSender construction to fail with an NPE.

Modification:

Add an end-to-end test that verifies DEFAULT mode deprioritizes a failed IP on subsequent connections using a multi-IP host and fresh connections. Reject negative cooldown periods with IllegalArgumentException while continuing to allow null (reset to the default) and Duration.ZERO. Guard NettyRequestSender against a null cooldown period when the feature is enabled, leaving the cooldown disabled instead of failing client construction.

Result:

The failed-IP cooldown behavior in DEFAULT mode is now covered end-to-end, ensuring regressions are detected. Invalid negative configuration now fails fast, and inconsistent custom configurations no longer cause client construction to fail.