run ensure_flag_arg_type for short flag values (#14074)

Closes #13654

# User-Facing Changes

- Short flags are now fully type-checked,
  including null and record signatures for literal arguments:

```nushell
def test [-v: record<l: int>] {};
test -v null # error
test -v {l: ""} # error

def test2 [-v: int] {};
let v = ""
test2 -v $v # error
```

- `polars unpivot` `--index`/`--on` and `into value --columns`
now accept `list` values

crates/nu-command/src/conversions/into/value.rs

@@ -18,7 +18,7 @@ impl Command for IntoValue {
             .input_output_types(vec![(Type::table(), Type::table())])
             .named(
                 "columns",
-                SyntaxShape::Table(vec![]),
+                SyntaxShape::List(Box::new(SyntaxShape::Any)),
                 "list of columns to update",
                 Some('c'),
             )