Fix mixed-type addition chain incorrectly treated as pure string concatenation

[Copilot]

The StringConcatenationExpression optimization introduced in 4.6.0 incorrectly flattened any left-recursive + chain containing a string literal — regardless of position — into a single string concat pass. This broke left-to-right numeric evaluation when numeric operands preceded the string literal.

engine.Evaluate("2.0 + 3.0 + 'm'"); // returned "23m", expected "5m"
engine.Evaluate("1 + 2 + 3 + '4'"); // returned "1234", expected "64"
engine.Evaluate("'m' + 2 + 3");     // returned "m23", still correct
engine.Evaluate("2 + 'm' + 3");     // returned "2m3", still correct

Changes

• JintBinaryExpression.cs — TryBuildStringConcatenation now only creates a StringConcatenationExpression when the string literal appears at operand index 0 or 1. A string at index ≥ 2 means the first operation (operands[0] + operands[1]) may be numeric addition, so flattening is unsafe. Adds a defensive operands.Count < 2 guard with an explanatory comment.
• StringTests.cs — Adds MixedTypeAdditionShouldEvaluateLeftToRight regression test covering the reported case and related variants.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Wrong evaluation of expressions with mixed types</issue_title>
<issue_description>This applies to Jint versions 4.6.0 to 4.6.3 running on .NET 8.0 to .NET 10.

Operating system used for testing Win 11.

The following expression is evaluated wrong:

2.0 + 3.0 + 'm' -> '23m' (expected: '5m')

Jint version 4.5.0 worked as expected (same result as in Firefox and Chrome).

Sample code:

			using Engine engine = new Engine((opt) =>
			{
				opt.Culture(new CultureInfo("en-US"));
			});

			JsValue val = engine.Evaluate("2.0 + 3.0 + 'm'");

			Console.WriteLine(val.ToString());
```</issue_description>

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

<comments>
</comments>

• Fixes Wrong evaluation of expressions with mixed types #2336

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