[TS] Handle null == undefined

[Lipen]

This PR fixes TsBinaryOperator.Eq::onRef for two heap refs which can represent null or undefined JS values, which are actually loosely equal (null == undefined, but not null === undefined), despite having different addresses. Other objects are compared via refs in JS, which is correctly translated to mkHeapRefEq.

[Copilot]

Pull Request Overview

This PR implements proper handling of JavaScript's loose equality operator (==) for null and undefined values, ensuring that null == undefined evaluates to true while maintaining correct behavior for other object comparisons.

• Adds comprehensive test coverage for null/undefined equality scenarios
• Refactors the TsBinaryOperator.Eq::onRef method to correctly handle the special case where null == undefined
• Updates existing equality resolution logic to use dedicated helper methods

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
usvm-ts/src/test/resources/samples/operators/Equality.ts	Adds test cases for null/undefined equality comparisons
usvm-ts/src/test/resources/samples/lang/Undefined.ts	Adds test method for checking undefined or null values
usvm-ts/src/test/resources/samples/lang/Null.ts	Adds test method for checking null or undefined values
usvm-ts/src/test/kotlin/org/usvm/samples/operators/Equality.kt	Adds Kotlin test cases for null/undefined equality scenarios
usvm-ts/src/test/kotlin/org/usvm/samples/lang/Undefined.kt	Adds test for undefined or null value detection
usvm-ts/src/test/kotlin/org/usvm/samples/lang/Null.kt	Adds test for null or undefined value detection and fixes import
usvm-ts/src/main/kotlin/org/usvm/machine/operator/TsBinaryOperator.kt	Implements proper null/undefined equality logic and refactors equality resolution

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