Function normalizeHandleList

Normalise any handle-list-like input into a plain array.

Priority order:

1. Convert() is honoured first — fast path for FbMetadbHandleList.
2. toArray() is honoured next for ad-hoc collection wrappers.
3. Plain Array short-circuits.
4. Array-like with length (e.g. arguments / NodeList).

Returns [] for unrecognised / falsy input.