Allow not() to compose with and()/or() (undefined-safe)

[J0su3Code]

Summary

Fixes #1818.

and() and or() are typed to return SQL | undefined (they drop
undefined conditions and return undefined when nothing is left).
not(), however, only accepted SQLWrapper, so composing them:

db.query.users.findMany({
  where: not(and(ilike(users.name, '%asdf%'))),
})

failed to type-check with:

TS2345: Argument of type 'SQL | undefined' is not assignable to parameter of type 'SQLWrapper'.
  Type 'undefined' is not assignable to type 'SQLWrapper'.

and forced a manual as SQLWrapper cast on the inner expression.

Change

Add an overload to not() so it also accepts SQLWrapper | undefined
and returns SQL | undefined, passing undefined straight through — the
same way and() / or() already ignore undefined conditions.

export function not(condition: SQLWrapper): SQL;
export function not(condition: SQLWrapper | undefined): SQL | undefined;
export function not(condition: SQLWrapper | undefined): SQL | undefined {
  return condition === undefined ? undefined : sql`not ${condition}`;
}

The original overload is kept, so not(eq(...)) still returns SQL
(not SQL | undefined). No existing call site changes its inferred type,
so this is non-breaking.

Tests

Added type-test coverage for not(and(...)) and not(or(...)) in the
allOperators block of the pg, mysql, sqlite, singlestore and geldb
type-tests/*/select.ts files. These lines fail to compile on main
(the exact TS2345 above) and compile with this change.

Note on conventions

This is a core, dialect-agnostic operator change, so it doesn't map to a
single [<dialect>]: tag — the type-tests are updated for every dialect
instead.