fix(orm): restore lazy $onUpdate evaluation in buildUpdateSet

[tsushanth]

Closes #5780.

Summary

The 0.45.x refactor of buildUpdateSet introduced eager evaluation of the $onUpdate callback for every column that defines one — including columns whose value is explicitly supplied in the caller's set object, where the callback's return value is then discarded via ??. Side-effecting callbacks (throwing, logging, reading external state) now run on every UPDATE, which is a breaking regression from the 0.44.6 contract where the callback only fired when the column was absent.

Fix

Restore lazy evaluation while keeping the 0.45.x SQL-type check that motivated the refactor:

const onUpdateFnResult =
  set[colName] !== undefined ? undefined : col.onUpdateFn?.();
const value = set[colName]
  ?? (is(onUpdateFnResult, SQL) ? onUpdateFnResult : sql.param(onUpdateFnResult, col));

Applied identically across all five dialects that carry the same buildUpdateSet shape:

• drizzle-orm/src/pg-core/dialect.ts
• drizzle-orm/src/mysql-core/dialect.ts
• drizzle-orm/src/sqlite-core/dialect.ts
• drizzle-orm/src/singlestore-core/dialect.ts
• drizzle-orm/src/gel-core/dialect.ts

The buildInsertQuery paths (around line ~548 in each file) are already lazy by construction — they're gated on colValue === undefined, so the INSERT default fallback path is untouched.

Tests

drizzle-orm/tests/on-update.test.ts (new):

• pg-core: $onUpdate callback that throws Error('should not be called when column is explicitly set'). The first test exercises an UPDATE with the column explicitly provided in set and asserts no throw + the expected SQL.
• pg-core: A counting $onUpdate callback to confirm the absent-column path still invokes it exactly once and the value flows through to params.

Verified that the first test fails on the buggy code (the callback throws) and passes after the fix. All 568 existing unit tests pass.

I scoped the regression test to pg-core to keep the diff focused; the identical fix in the other four dialects is auditable by inspection and the integration test suite under integration-tests/tests/ exercises them against live DBs.