Add unit tests for Action logic

[JosiahWI]

I authored these tests with Claude Sonnet and Claude Opus. They are part of a suite of unit tests I am generating with AI, aimed at the event system. The Action interface doesn't have much logic, so most of what's tested here is pretty simple, but there are a few tests related to cancellation that serve as good documentation/examples of how to use actions appropriately.

[Copilot]

Pull request overview

Adds a new Catch2 unit test executable covering the Action contract in the inkevent event system, with emphasis on assignment semantics (continuation/mutex binding) and cancellation behavior (including cancellation of scheduled events).

Changes:

• Introduces test_Action.cc with unit tests validating Action’s observable state transitions (operator=, cancel, cancel_action) and sentinel encodings (MAKE_ACTION_RESULT, ACTION_RESULT_DONE, ACTION_IO_ERROR).
• Adds a multithreaded test that schedules and cancels an event before its deadline to verify no dispatch occurs.
• Wires the new unit test into the inkevent CMake test targets.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/iocore/eventsystem/unit_tests/test_Action.cc	New Catch2 tests exercising Action assignment/cancellation behavior and action-result sentinels, plus a scheduled-event cancellation scenario.
src/iocore/eventsystem/CMakeLists.txt	Adds test_Action executable and registers it as a Catch2 test.