Fall back to plain SO_KEEPALIVE when jdk.net module is absent

[meteaksoyy]

ExtendedKeepAliveSupport.ExtendedNioSocketOptions referenced jdk.net.ExtendedSocketOptions through a class literal, so on runtimes without the jdk.net module (for example custom jlink images) class initialization failed with NoClassDefFoundError and crashed application startup. Since #3669 made extended keep-alive the default, this happens for every affected application.

The class is now resolved via Class.forName, so a missing module results in a catchable ClassNotFoundException and Lettuce degrades to plain SO_KEEPALIVE, restoring the pre-7.5.0 behavior. LinkageError is caught alongside ReflectiveOperationException to cover partially available modules, and unavailability is logged at debug level.

New ExtendedKeepAliveSupportUnitTests includes a reproducer that loads the class in a classloader refusing jdk.net.*, simulating a runtime image without the module. It fails with the reported NoClassDefFoundError against the previous code and passes with this change.

Fixes #3862

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Note

Low Risk
Narrow change to optional NIO keep-alive discovery with graceful degradation; behavior on full JDKs unchanged when jdk.net is present.

Overview
Fixes startup crashes on custom jlink (or other) runtimes that omit the jdk.net module, where ExtendedNioSocketOptions used to pull in jdk.net.ExtendedSocketOptions via a compile-time reference and could throw NoClassDefFoundError during static init.

ExtendedNioSocketOptions now loads jdk.net.ExtendedSocketOptions with Class.forName, drops the direct import, and treats ReflectiveOperationException and LinkageError as "extended options unavailable" (debug log, null socket options). isSupported() / isAvailable() then report false and Lettuce falls back to plain SO_KEEPALIVE instead of failing startup.

Adds ExtendedKeepAliveSupportUnitTests: standard JDK 11+ availability, isSupported() must not throw, and a jdk.net.*-blocking classloader reproducer that expects isAvailable() == false without NoClassDefFoundError.

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