Fix RegExp literal NullReferenceException in getters parsed via ParseExpression

[Copilot]

Fixed a NullReferenceException that occurred when using RegExp literal notation in getters parsed via Parser().ParseExpression(). The issue manifested in RegExpPrototype.RegExpBuiltinExec when the regex object was null.

Problem

When expressions containing regex literals are parsed individually (not as part of a full script preparation), the regex compilation step was skipped, leaving the Regex object uninitialized. This caused a null reference exception when the regex was later executed.

var engine = new Engine();
var obj = new JsObject(engine);
var getterFunction = (IFunction)new Parser().ParseExpression(
     "function() { return 'test'.match(/[a-z]/)?.[0]; }" // Previously threw NullReferenceException
);
var getter = new ScriptFunction(engine, getterFunction, true);
obj.FastSetProperty("name", new GetSetPropertyDescriptor(getter, null, true, false));

var result = engine.GetValue("customObj").AsObject().Get("name"); // Would fail here

Solution

Modified JintLiteralExpression.ResolveValue() to perform on-demand regex compilation when regExpParseResult.Regex is null. The fix uses the same Tokenizer.AdaptRegExp() method that's used in RegExpConstructor to ensure consistent behavior.

The solution:

1. Checks if the regex object is null after parsing
2. Compiles the regex using the engine's parser options if needed
3. Maintains proper error handling for invalid regex patterns
4. Preserves all existing functionality for prepared scripts and direct evaluation

Testing

• Original failing case now works correctly
• All existing regex functionality remains unaffected
• Complex regex patterns (global, ignoreCase, multiline, named groups, unicode) work properly
• Both literal notation (/pattern/flags) and constructor notation (new RegExp()) continue to work
• Edge cases with invalid patterns are handled appropriately

Fixes #2161.

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