Set-operation orderBy() ignores casing config (pg + sqlite), producing a non-existent column reference

### Description
With `casing: 'snake_case'` (columns declared without an explicit DB name), ordering a set operation (`union`/`intersect`/`except`/…) by a column emits the JS property name in `ORDER BY` instead of the cased DB column name. The SELECT list is cased correctly, so `ORDER BY` references a column that isn't in the result set — the query fails at runtime (Postgres: `column "firstName" does not exist`). Affects **pg** (bare and `asc()`/`desc()`-wrapped) and **sqlite** (bare-column path). mysql/singlestore are unaffected.

### Steps to reproduce
```ts
import { pgTable, text, PgDialect, QueryBuilder } from 'drizzle-orm/pg-core';
import { asc, union } from 'drizzle-orm';

const users = pgTable('users', { firstName: text() }); // no explicit db name
const dialect = new PgDialect({ casing: 'snake_case' });
const qb = new QueryBuilder(dialect);

union(qb.select().from(users), qb.select().from(users)).orderBy(asc(users.firstName));
```
Generated SQL:
```
observed: (select "first_name" from "users") union (select "first_name" from "users") order by "firstName" asc
expected: (select "first_name" from "users") union (select "first_name" from "users") order by "first_name" asc
```
A plain `select().orderBy(asc(users.firstName))` is correct (`order by "users"."first_name" asc`); only set-operation ordering is affected.

### Root cause
`buildSetOperationQuery` emits the raw column `.name` instead of `casing.getColumnCasing(...)`:
- `drizzle-orm/src/pg-core/dialect.ts:483` (bare column) and `:489` (sql-wrapped chunk)
- `drizzle-orm/src/sqlite-core/dialect.ts:478` (bare column)

The correct pattern is already used in `mysql-core/dialect.ts:528,536`, `singlestore-core/dialect.ts`, and `sqlite-core/dialect.ts:485`.

### Suggested fix
Replace those three sites with `this.casing.getColumnCasing()` (the column object, matching mysql). After the change, all set-operation `ORDER BY` clauses emit `"first_name"`, and plain selects are unchanged.

Start at buildSetOperationQuery in drizzle-orm/src/pg-core/dialect.ts (lines 483 and 489) and drizzle-orm/src/sqlite-core/dialect.ts (line 478), comparing the existing casing calls in mysql-core/dialect.ts and sqlite-core/dialect.ts. Verify set-operation ORDER BY output for snake_case columns matches the expected first_name SQL for bare and wrapped ordering, while plain selects remain unchanged.

Written by the indexing model from the issue text.