Fix(macros): @FILTER with a single non-array argument

[chuenchen309]

Description

@FILTER's docstring says "The first argument can be an Array or var args can be used" — the same contract @EACH and @REDUCE state. All three route their arguments through _norm_var_arg_lambda, whose single-item branch returns a bare expression rather than a list:

expressions = (
    item.expressions if isinstance(item, (exp.Array, exp.Tuple))
    else [item.this] if isinstance(item, exp.Paren)
    else item          # <- bare expression
)

@EACH (macros.py:714) and @REDUCE (:764) absorb that with ensure_collection(). @FILTER (:792) iterates it directly, so a lone argument raises:

@EACH(a, x -> x + 1)        -> SELECT a + 1
@REDUCE(a, (x, y) -> x + y) -> SELECT a
@FILTER(a, x -> x > 1)      -> MacroEvalError: 'Column' object is not iterable

An explicit array already worked for all three, so this only affects the lone-argument shape the docstring advertises.

Test Plan

Added two cases to test_macro_functions, next to the existing array-based @FILTER case so the two shapes sit together:

• @SQL(@REDUCE(@FILTER(300, x -> x > 250), (x,y) -> x + y)) → 300 (mirrors the existing [100, 200, 300, 400] → 700 case)
• select @FILTER(a, x -> x > 1) → SELECT a

Both fail on main and pass with this change. tests/core/test_macros.py is 140 passed with the fix, 138 passed / 2 failed without it. tests/core/test_macros.py tests/core/test_dialect.py together: 296 passed.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO

On make fast-test: I ran ruff, ruff-format and the migrations hook (all pass) plus the test files above, but not the full suite. mypy reports only Cannot find implementation or library stub for module named "sqlmesh._version" in four unrelated files — that's the setuptools-scm generated module missing from my worktree (it's gitignored), not something this diff touches.

Disclosure: written with Claude Code, which I see this repo already uses. I verified the reproduction, the sibling comparison, and the test red/green myself.