test: unit tests for the least covered core packages (metrics, ack, misc helpers)

[devin-ai-integration]

Description

JaCoCo run over the full netty-socketio-core suite (all 113 test classes, Docker/Testcontainers included) showed these packages/classes as the weakest, ignoring the Redis stream/reliable stores which have no test class matching the surefire **/*Test.java include (DistributedRedissonClusterSuite):

target	before	after
metrics package	2.8%	100% (MicrometerSocketIOMetrics 99.3%)
ack package	24.8%	~95% (AckManager 91%, AckSchedulerKey/AckEntry 100%)
BroadcastAckCallback / AckCallback / VoidAckCallback / MultiTypeAckCallback	0%	100% (BroadcastAckCallback 95.7%)
MultiTypeArgs	14%	100%
JsonSupportWrapper	31.5%	100%
HandshakeData	40%	100%
WrongUrlHandler	10.8%	100%
store.event.PublishConfig / PublishMode / EventStoreType	0%	100%
listener.ExceptionListenerAdapter / DefaultExceptionListener	30% / partial	100%

No production code was touched; JaCoCo was only wired in locally for measurement and is not part of this diff.

Type of Change

• Test improvements

Changes Made

• ack/AckManagerTest — ack index allocation (registerAck returns initAckIndex-relative indexes), single vs. MultiTypeAckCallback dispatch on onAck, timeout scheduling/cancellation and onDisconnect timing out pending callbacks, using a recording CancelableScheduler stub that can fire scheduled callbacks manually
• ack/AckSchedulerKeyTest — equals/hashCode across differing type, session and index, including inequality with the plain SchedulerKey
• metrics/MicrometerSocketIOMetricsTest — counters/timers/gauges against a SimpleMeterRegistry, empty namespace mapped to default, non-positive durations not recorded, room members gauge clamped at 0, factory + noop implementation
• metrics/NamespaceMetersTest — meter registration and the periodic (5s) publish window of the unknown-event-name HLL estimate
• AckCallbackTest — callback hierarchy plus MultiTypeArgs out-of-bounds-safe accessors and the BroadcastAckCallback loopFinished()/counter interplay that gates onAllSuccess
• JsonSupportWrapperTest, HandshakeDataTest, handler/WrongUrlHandlerTest (via EmbeddedChannel: 400 + release + close), store/event/PublishConfigTest, listener/ExceptionListenerTest

Testing

• All existing tests pass
• New tests added for new functionality
• Tests pass locally with mvn test

76 new tests, all green locally with checkstyle/PMD/license checks enabled.

Link to Devin session: https://app.devin.ai/sessions/727bd5be6dd5437caf53f6ff46651322
Requested by: @sanjomo

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.