JS: Introduce helper functions for event listeners

[osipxd]

Subsystem
JS/WasmJS client

Motivation
The existing approach was error-prone: it was easy to forget to call removeEventListener for one-time listeners, and the only correct implementation required a lateinit var to hold the callback reference so it could be removed inside itself.

Solution
Introduces two internal helpers in the web source set (shared between js and wasmJs):

• EventTarget.addEventListener<T>(vararg events, listener) — type-safe wrapper that captures the callback internally and returns a DisposableHandle, eliminating the need to keep a separate lambda reference for cleanup.
• EventTarget.addOneTimeEventListener<T>(vararg events, listener) — registers a listener that automatically removes itself after the first matching event fires, replacing the lateinit var pattern.

Refactors JsClientEngine, WasmJsClientEngine, and JsWebSocketSession (both js and wasmJs) to use these helpers.

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• Commit unit tests in branch osipxd/typesafe-js-event

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[bjhham]

Good call 👍