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_plugin_polars/src/dataframe/command/data/unpivot.rs

@@ -31,13 +31,13 @@ impl PluginCommand for UnpivotDF {
         Signature::build(self.name())
             .required_named(
                 "index",
-                SyntaxShape::Table(vec![]),
+                SyntaxShape::List(Box::new(SyntaxShape::Any)),
                 "column names for unpivoting",
                 Some('i'),
             )
             .required_named(
                 "on",
-                SyntaxShape::Table(vec![]),
+                SyntaxShape::List(Box::new(SyntaxShape::Any)),
                 "column names used as value columns",
                 Some('o'),
             )