Merge succeeds with invalid generated-column expression after source column rename

### Description

A three-way schema merge can report success with zero conflicts while producing a table whose generated-column expression references a column name that no longer exists.

One branch adds a generated column based on `a`; the other branch renames `a` to `base`. The merge keeps the renamed column but does not rewrite or reject the generated expression, so even `SHOW CREATE TABLE` fails afterward.

### Reproduction

Start in a new directory:

```sh
dolt init --name oracle --email oracle@test

dolt sql <<'SQL'
CREATE TABLE t(id INTEGER PRIMARY KEY, a INT);
INSERT INTO t VALUES (1, 5);
CALL dolt_commit('-Am', 'ancestor');
CALL dolt_branch('feat');

CALL dolt_checkout('feat');
ALTER TABLE t ADD COLUMN doubled INT GENERATED ALWAYS AS (a * 2) VIRTUAL;
CALL dolt_commit('-Am', 'add generated');

CALL dolt_checkout('main');
ALTER TABLE t RENAME COLUMN a TO base;
CALL dolt_commit('-Am', 'rename source');

CALL dolt_merge('feat');
SHOW CREATE TABLE t;
SELECT id, base, doubled FROM t;
SQL
```

### Actual behavior

`dolt_merge` reports a successful merge with no conflicts:

```text
| fast_forward | conflicts | message |
| 0 | 0 | merge successful |
```

The next statement fails:

```text
error on line 1 for query SHOW CREATE TABLE t: column "a" could not be found in any table in scope
```

The merged schema retained the generated expression's reference to `a`, even though the merged source column is named `base`.

### Expected behavior

The merge must not report success and install an invalid schema. It should either:

- produce a valid merged schema, rewriting the generated expression to reference `base`; or
- return a handled schema conflict and leave the working state valid.

### Environment

- Dolt 2.2.2
- macOS arm64

Run the supplied dolt_merge SQL reproduction first, including SHOW CREATE TABLE and the SELECT after the merge. Trace the three-way schema merge and generated-column handling for the rename and added-column branches. Done means the merge either rewrites the expression to base or reports a handled schema conflict without installing an invalid schema.

Written by the indexing model from the issue text.