[Tech Debt] [runtime] Add comprehensive tests for ActionExecutionOperator manager classes

[weiqingy]

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Description

Summary

PR #546 introduced 5 focused manager classes by decomposing ActionExecutionOperator. Minimal contract tests (4 files, 7 tests) were added to keep that PR reviewable. Heavier coverage was explicitly deferred and is tracked here.

Deferred Coverage

Manager	Missing Coverage
OperatorStateManager	Flink-harness-based tests (real StateInitializationContext, keyed state, operator state backend)
DurableExecutionManager	Full snapshot + recovery cycle; notifyCheckpointComplete pruning behaviour
EventRouter	Listener notification on event processed; watermark draining order under concurrent keys
ActionTaskContextManager	Full memory-context lifecycle (create → transfer → complete → cleanup)

Notes

• The contract tests added in PR [runtime] Refactor ActionExecutionOperator into focused manager classes #546 (e2e4e03) cover the core promise of each manager but use mocks and simple stubs.
• Flink-harness tests require AbstractStreamOperatorTestHarness and real Flink state backends — heavier setup, better suited to a focused follow-up (keep PR [runtime] Refactor ActionExecutionOperator into focused manager classes #546 reviewable).

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