[BUG]: drizzle-kit push tries to DROP Neon system roles despite provider: "neon"

### Report hasn't been filed before.

- [x] I have verified that the bug I'm about to report hasn't been filed before.

### What version of `drizzle-orm` are you using?

1.0.0-rc.4

### What version of `drizzle-kit` are you using?

1.0.0-rc.4

### Other packages

_No response_

### Describe the Bug

On a Neon database, `drizzle-kit push` tries to drop Neon's internal roles (`cloud_admin`, `neon_service`), even though my schema has no `pgRole` at all and the config sets `entities: { roles: { provider: 'neon' } }`. Neon denies the drop, so push always fails:

```
┌── query error: permission denied to drop role

DROP ROLE "cloud_admin";
└──
```

Two problems:

1. Role diffing runs even when the schema declares no roles. It also happens with no `entities` block in the config at all.
2. The `provider: 'neon'` exclusion list doesn't cover Neon's current system roles. Adding `exclude: ['cloud_admin']` just fails on the next one, `DROP ROLE "neon_service"`.

This didn't happen on earlier 1.0.0 betas — probably because #5896 (config-file `entities` silently stripped, fixed in rc.4) was masking it.

**Steps to reproduce**

1. Any Neon project; a schema with normal tables and no `pgRole`.

2. Config:

```ts
import { defineConfig } from "drizzle-kit";

export default defineConfig({
schema: "./src/schema/index.ts",
out: "./migrations",
dialect: "postgresql",
dbCredentials: { url: process.env.DATABASE_URL! },
entities: {
roles: {
provider: "neon",
},
},
});
```

3. `drizzle-kit push` → fails on `DROP ROLE "cloud_admin"`.

4. Add `exclude: ['cloud_admin']` → fails on `DROP ROLE "neon_service"`.

**Workaround:** `entities: { roles: false }` — push then completes with `No changes detected`.

**Expected:** with `provider: 'neon'` (or no roles in the schema), push shouldn't touch Neon's system roles and should report `No changes detected` on an in-sync schema.

I'm using `push` for local development (`generate` + `migrate` in production), so I can't push any schema changes right now.

Start with the drizzle-kit push role-diffing path and the handling of entities.roles.provider for Neon, reproducing the report against a Neon database with a schema containing no pgRole entries. Done means an in-sync schema reports No changes detected without attempting to drop cloud_admin or neon_service, both with the Neon provider and without a roles configuration.

Written by the indexing model from the issue text.