$expr inside relation filters can't reference the filtered model (missing FROM-clause entry) #2863

Description and expected behavior

$expr types its expression builder with the filtered model as the scope (ExpressionBuilder<DB, Model>), so qualified references like 'User.email' type-check. That holds for top-level filters, where the table is selected as "User". Relation filters, though, select the related model under a generated alias (e.g. "User" as "$$_Post$author"), so the same reference fails at runtime:

// works - top-level filter, the table is selected as "User"
await db.user.findMany({
    where: { $expr: (eb) => eb('User.email', 'like', '%@zenstack.dev') },
});

// type-checks, but fails with: missing FROM-clause entry for table "User"
await db.post.findMany({
    where: { author: { $expr: (eb) => eb('User.email', 'like', '%@zenstack.dev') } },
});

Unqualified references (eb('email', ...)) work because they resolve against the innermost table. They break down once the expression contains a subquery with a same-named column, e.g. a correlated subquery that needs the outer row's id, and $expr has no way to learn the real alias.

Expected: $expr gets the alias of the filtered model so expressions can qualify references, the same way computed field implementations already receive modelAlias in their context.

Proposed (non-breaking) fix: pass a second context argument with modelAlias to $expr:

await db.post.findMany({
    where: {
        author: {
            $expr: (eb, { modelAlias }) => eb(eb.ref(`${modelAlias}.email`), 'like', '%@zenstack.dev'),
        },
    },
});

I have a PR ready for this.

Environment (please complete the following information):

• ZenStack version: 3.9.0 (the $expr call site is unchanged on dev)
• Database type: PostgreSQL
• Node.js/Bun version: 22.23.2
• Package manager: pnpm

Additional context

The $expr callback is invoked in BaseCrudDialect.buildFilter as _where['$expr'](this.eb), while the modelAlias it needs is already in scope there.