Fix InvalidCastException when a property getter is a bound function

[Copilot]

ObjectInstance.UnwrapFromGetter and PrivateGet both cast the getter directly to Function.Function, but BindFunction extends ObjectInstance directly (not Function) and only implements ICallable. This causes an InvalidCastException when accessing a property whose getter was created via .bind().

const holder = { x: 42, getter() { return this.x; } };
const target = {};
Object.defineProperty(target, "prop", { get: holder.getter.bind(holder) });
target.prop; // throws InvalidCastException before fix, returns 42 after

Changes

• ObjectInstance.UnwrapFromGetter: Replace direct (Function.Function) cast with ICallable. Route the call through engine.Call(callable, thisObject, Arguments.Empty, null) (engine accessed via (ObjectInstance)getter) — this handles all callable types (BindFunction, JsProxy, etc.) while preserving call-stack management required by the debugger.
• ObjectInstance.PrivateGet: Same fix; uses the already-available _engine instance field.
• FunctionTests: Add regression test BoundFunctionCanBeUsedAsPropertyGetter.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Object getter can throw InvalidCastException for BindFunction</issue_title>
<issue_description>When an object property is defined as a getter, which in turn is a bound function, accessing the property results in InvalidCastException in ObjectInstance.UnwrapFromGetter, because getter variable is Jint.Native.Function.BindFunction instead of Jint.Native.Function.Function.

Explicit test for BindFunction is not ideal and perhaps ICallable should be used, but I'm not sure how to properly pass it to Engine.

Test code to reproduce the behavior:

Console.WriteLine(new Engine().Evaluate("""
	const holder = {
		x: 42,
		getter() { return this.x; }
	};

	const target = {};

	Object.defineProperty(target, "prop", { get: holder.getter.bind(holder) });

	target.prop;
"""));

Before fix: throws InvalidCastException
After fix: prints out 42</issue_description>

<agent_instructions>You must check the original PR fix attempt and the code review notes when deriving a fix. Find spec-compliant and logical solution.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@lahma See discussion and code review at https://github.com//pull/2299

• Fixes Object getter can throw InvalidCastException for BindFunction #2300

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