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

@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 and @reduce absorb that with ensure_collection(); @filter 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

Wrap the items in ensure_collection() so @filter matches its siblings. An
explicit array was already working and is unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: chuenchen309 <48723787+chuenchen309@users.noreply.github.com>

Author: chuenchen309

sqlmesh/core/macros.py

@@ -789,7 +789,7 @@ def filter_(evaluator: MacroEvaluator, *args: t.Any) -> t.List[t.Any]:
     """
     *items, func = args
     items, func = _norm_var_arg_lambda(evaluator, func, *items)  # type: ignore
-    return list(filter(lambda arg: evaluator.eval_expression(func(arg)), items))
+    return list(filter(lambda arg: evaluator.eval_expression(func(arg)), ensure_collection(items)))
 
 
 def _optional_expression(

tests/core/test_macros.py

@@ -358,6 +358,18 @@ def test_ast_correctness(macro_evaluator):
             "700",
             {},
         ),
+        # @FILTER documents "The first argument can be an Array or var args can be
+        # used", like @EACH and @REDUCE, so a lone item must work too.
+        (
+            """@SQL(@REDUCE(@FILTER(300, x -> x > 250), (x,y) -> x + y))""",
+            "300",
+            {},
+        ),
+        (
+            """select @FILTER(a, x -> x > 1)""",
+            "SELECT a",
+            {},
+        ),
         (
             """select @EACH([a, b, c], x -> x and @SQL('@y'))""",
             "SELECT a AND z, b AND z, c AND z",