[PandaHUN777]

Suggest List.concat [a] [b] when list operands are used with numeric +, while retaining the existing string concatenation guidance. Add a regression test for list addition and keep the generic diagnostic for unrelated numeric mismatches.

Closes #134

Validation:

• cargo fmt --check
• cargo clippy --all-targets
• cargo test

[simontreanor]

Thanks! The check is in the right place and the regression test is good. My only concern is the wording of the hint:

use `List.concat [a] [b]`

This reads as "wrap each side in a one-element list". Someone copying it for [1, 2] + [3] could reasonably write List.concat [[1, 2]] [[3]]. Could it say use `List.concat xs ys` instead? That matches the existing use `String.concat a b` hint next to it (and the assertion in plus_on_lists_hints_at_list_concat would change to match).

A heads-up as well: #169 also renames string_concat_hint, for the *-on-strings hint. I'm merging that one first, so this branch will need a rebase onto main. Folding the list arm into whatever the function is called by then (string_operator_hint from #169, or a more general name that covers both) is fine either way.

[simontreanor]

Thanks again for this! I've pushed the change from my review so it can make the 0.10.0 release: the hint now says List.concat xs ys. I also merged main in. #169 had reworked the same helper for * on strings, so your list case and that one now share a single operator_hint. Merging once CI is green.