ALTER COLUMN ... SET NOT NULL fails on text columns with a secondary index

`ALTER TABLE ... ALTER COLUMN ... SET NOT NULL` fails on a text column that has a secondary index, with a MySQL-dialect error:

```sql
CREATE TABLE t1 (pk integer PRIMARY KEY, value text);
CREATE INDEX t1_value_idx ON t1 (value);
INSERT INTO t1 VALUES (1, 'one');

ALTER TABLE t1 ALTER COLUMN value SET NOT NULL;
ERROR: blob/text column 'value' used in key specification without a key length
```

The statement works when the column has no secondary index. The error comes from GMS's `ValidateModifyColumn`, which applies MySQL's key-prefix-length rule to text/blob columns on any column modification; Postgres text columns are indexable without a key length, so the rule shouldn't apply to Doltgres.

Reproduce the SQL example, then locate GMS's ValidateModifyColumn and inspect how it applies the MySQL key-prefix-length rule to text columns. Verify the change with a regression case covering a secondary-indexed text column, and confirm ALTER COLUMN ... SET NOT NULL succeeds without the key-length error.

Written by the indexing model from the issue text.