fix: splitUnit returns original unit when called as a method with array parts (#3644)

[MaksZhukov]

What

Fixes the splitUnit method-call form in the expression parser, which silently returned the original unit unchanged.

(1 m).splitUnit([ft, in])      // before: 1 m        after: 3 ft,3.3700787401574765 in
(1 m).splitUnit(["ft", "in"])  // before: 1 m        after: 3 ft,3.3700787401574765 in

Closes #3644.

Why

Unit.prototype.splitUnit iterates over parts.length. When splitUnit is invoked as a method in the expression parser (e.g. (1 m).splitUnit([ft, in])), the parser dispatches the call directly to the prototype method via getSafeMethod/fn.apply(object, values), passing the array literal as a Matrix (the default matrix: 'Matrix' config) rather than a plain Array.

A Matrix has no .length property, so parts.length is undefined, the for loop never executes, and the method falls through to returning [this] — the original, unsplit unit.

The function form splitUnit(1 m, [ft, in]) was unaffected because it goes through the typed-function 'Unit, Array' signature, which converts the Matrix to an Array before reaching the prototype method.

Fix

Normalize parts to an Array (via toArray()) when it is a Matrix, so the method-call form behaves identically to the function-call form. Minimal change, no new dependencies.

How to test

math.evaluate('(1 m).splitUnit([ft, in])').toString()     // '3 ft,3.3700787401574765 in'
math.evaluate('(1 m).splitUnit(["ft", "in"])').toString() // '3 ft,3.3700787401574765 in'

A regression test covering the Matrix/method-call form was added to test/unit-tests/type/unit/function/splitUnit.test.js. The full unit-test suite (npx mocha test/unit-tests) passes (6653 passing) and eslint is clean on the changed files.

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