`BIT` grouping column is returned as integer after a mergeable CTE or derived table

MariaDB changes the result type of a `BIT` column when the column passes through a
mergeable CTE or derived table and is then used as an outer `GROUP BY` key.

The direct query returns the grouping column with the MariaDB protocol type `BIT`
and binary values such as `b'\xf5'`. The equivalent CTE or derived-table query
returns the same column with protocol type `LONG` and integer values such as
`245`.

The numeric bit patterns are unchanged, but the SQL result metadata and the
client-visible values are different:

```text

Direct query:        b'\x09', b'\xf0', b'\xf5'  (BIT)

CTE/derived query:   9,       240,      245      (LONG)

```

This case does not use `BIT_XOR` or another bit aggregate. It affects an ordinary
`BIT(8)` column selected as a grouping key, so it is distinct from the
`BIT_XOR` materialization type-change issue.

1. How to repeat

Create a self-contained test table:

```sql

DROP DATABASE IF EXISTS bit_group_type_test;

CREATE DATABASE bit_group_type_test;

USE bit_group_type_test;

CREATE TABLE t (

    id INT PRIMARY KEY,

    b BIT(8),

    payload VARBINARY(8)

);

INSERT INTO t VALUES

    (1, b'11110101', X'F5'),

    (2, b'00001001', X'09'),

    (3, b'11110000', X'F0');

```

Run the direct query:

```sql

SELECT b, MAX(payload) AS m

FROM t

GROUP BY b;

```

The result column `b` is exposed as `BIT` and a binary-capable client receives:

```text

b'\x09'

b'\xf0'

b'\xf5'

```

Now put the source columns in a mergeable CTE:

```sql

WITH x AS (

    SELECT b, payload

    FROM t

)

SELECT b, MAX(payload) AS m

FROM x

GROUP BY b;

```

The result column `b` is exposed as `LONG` and the same client receives:

```text

9

240

245

```

The equivalent derived-table form has the same problem:

```sql

SELECT b, MAX(payload) AS m

FROM (

    SELECT b, payload

    FROM t

) AS x

GROUP BY b;

```

1. Expected result

All three equivalent queries should expose the grouping column as `BIT(8)` and
return the same binary representation. Introducing a relational CTE or derived
table should not change the selected column's result type.

1. Actual result

The mergeable CTE and derived-table forms expose the grouping column as `LONG`.
Applications therefore receive Python integers instead of byte strings, despite
the source column remaining `BIT(8)`.