Add support for Strings

[Lipen]

This PR adds support for strings in TS machine.

[Copilot]

Pull Request Overview

Adds support for JavaScript string values in the TS machine, including type‐system, interpreter, context, and test updates.

• Introduces heap‐allocated string constants and updates typeof handling to return proper string sorts.
• Extends TsTypeSystem and TsInterpreter to recognize EtsStringType, and adds sample/tests for string behavior.
• Refactors existing samples and code (strict equality, variable renames, commented‐out heuristics) for consistency.

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
usvm-ts/src/test/resources/samples/types/StructuralEquality.ts	Renamed locals and added lint suppression
usvm-ts/src/test/resources/samples/types/ObjectUsage.ts	Changed parameter type to {} and used strict ===
usvm-ts/src/test/resources/samples/arrays/InputArrays.ts	Switched == to === for undefined checks
usvm-ts/src/test/resources/samples/Strings.ts	New sample class covering typeof on all JS basic values
usvm-ts/src/test/kotlin/org/usvm/util/TsTestResolver.kt	Added EtsStringType resolution, new helper import
usvm-ts/src/test/kotlin/org/usvm/util/TsMethodTestRunner.kt	Removed placeholder mapping for TsObject
usvm-ts/src/test/kotlin/org/usvm/util/Truthy.kt	Deleted obsolete isTruthy for TsObject
usvm-ts/src/test/kotlin/org/usvm/samples/types/ObjectUsage.kt	Updated property‐based testing for objectAsParameter
usvm-ts/src/test/kotlin/org/usvm/samples/arrays/InputArrays.kt	Adjusted generics and exception handling in array tests
usvm-ts/src/test/kotlin/org/usvm/samples/Strings.kt	Added full suite of Kotlin tests for Strings sample
usvm-ts/src/test/kotlin/org/usvm/samples/InstanceFields.kt	Refactored generics and removed null/undefined branch tests
usvm-ts/src/main/kotlin/org/usvm/machine/types/TsTypeSystem.kt	Included EtsStringType in object class type branches
usvm-ts/src/main/kotlin/org/usvm/machine/interpreter/TsInterpreter.kt	Refactored context usage, added string param constraints
usvm-ts/src/main/kotlin/org/usvm/machine/expr/TsExprResolver.kt	Replaced magic‐number heuristics with mkStringConstant
usvm-ts/src/main/kotlin/org/usvm/machine/TsContext.kt	Implemented mkStringConstant to allocate string objects
usvm-ts/src/main/kotlin/org/usvm/api/TsTest.kt	Removed TsObject from the sealed interface

Comments suppressed due to low confidence (6)

usvm-ts/src/test/resources/samples/types/StructuralEquality.ts:46

• [nitpick] The variable name x is not descriptive. Consider renaming it to obj or instance to clarify its purpose.

        const x: A = new B(11);

usvm-ts/src/test/resources/samples/types/ObjectUsage.ts:5

• [nitpick] The parameter name x is ambiguous. Consider renaming it to obj or inputObj for clarity.

    objectAsParameter(x: {}): number {

usvm-ts/src/test/kotlin/org/usvm/samples/types/ObjectUsage.kt:18

• The test no longer asserts the undefined input path yields -1. Consider adding an explicit branch to cover x == undefined returning -1.0.

            { x, r -> r.number == 42.0 },

usvm-ts/src/test/kotlin/org/usvm/samples/InstanceFields.kt:17

• The tests removed the branch covering undefined or null inputs producing exceptions. To maintain full coverage, reintroduce that case.

        discoverProperties<TsTestValue.TsClass, TsTestValue.TsNumber>(

usvm-ts/src/test/kotlin/org/usvm/util/TsTestResolver.kt:38

• The import isAllocated is never used in this file. You can remove it to keep imports tidy.

import org.usvm.isAllocated

usvm-ts/src/test/kotlin/org/usvm/util/TsTestResolver.kt:46

• The import TsInterpreter is not referenced in this file. Consider removing it to avoid unused imports.

import org.usvm.machine.interpreter.TsInterpreter