sql: support adding unique indexes including crdb_region on REGIONAL BY ROW tables via ALTER TABLE

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Currently, CockroachDB supports creating unique constraints on REGIONAL BY ROW tables that include the implicit crdb_region column at table creation time. However, attempting to add the same constraint after table creation via ALTER TABLE ... ADD CONSTRAINT ... UNIQUE (...) fails with an error about implicit partitioning columns and hash-sharded indexes.

Example:

```
CREATE TABLE transaction (
id UUID,
reference_id TEXT
) LOCALITY REGIONAL BY ROW;

ALTER TABLE transaction
ADD CONSTRAINT transaction_reference_id_unique
UNIQUE (crdb_region, reference_id);
```

Results in:

`ERROR: hash sharded indexes cannot include implicit partitioning columns from "PARTITION ALL BY" or "LOCALITY REGIONAL BY ROW"`

The same constraint, when defined in the CREATE TABLE statement, works correctly:

```
CREATE TABLE transaction (
id UUID,
reference_id TEXT,
UNIQUE (crdb_region, reference_id)
) LOCALITY REGIONAL BY ROW;
```

**Describe the solution you'd like**
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`ALTER TABLE ... ADD CONSTRAINT ... UNIQUE` (crdb_region, reference_id) should succeed on a `REGIONAL BY ROW` table — equivalently to defining the constraint at table creation time — because both paths conceptually produce the same locality-aware index.

Jira issue: CRDB-55339

Start by reproducing the CREATE TABLE and ALTER TABLE examples in the issue and compare how each creates the locality-aware unique index. Trace the SQL handling for ALTER TABLE ... ADD CONSTRAINT ... UNIQUE on REGIONAL BY ROW tables; done when the ALTER form accepts crdb_region and behaves like the equivalent table-creation form.

Written by the indexing model from the issue text.