InstallWizard: optionalFieldsAuto: all not recognized as configurable in getSubPages()

[dionlow]

Problem

When an object in amp.yaml declares optionalFieldsAuto: all (and no explicit optionalFields:, no requiredFields, no mapToName, no field mappings), the InstallWizard lands on the empty Fields sub-page with a message that nothing needs configuring — even though the user should be able to pick from auto-discovered customer fields on the Additional fields sub-page.

Repro

amp.yaml with an object like:

```yaml

• objectName: opportunity
  destination: testWebhook
  schedule: "*/10 * * * *"
  optionalFieldsAuto: all
  ```

Wizard shows the empty Fields page instead of the Additional fields picker.

Root cause

src/components/InstallWizard/steps/configure-objects/subPageUtils.ts:63-65 decides whether to add the `additional` sub-page using:

```ts
const hasOptionalFields = (obj.getOptionalFields("no-mappings")?.length ?? 0) > 0;
```

`getOptionalFields` (in `src/headless/manifest/useManifest.ts`) reads from `object.optionalFields` — the explicit YAML list. Auto-discovered fields from `optionalFieldsAuto: all` live in `object.allFieldsMetadata` (surfaced via `getCustomerFieldsForObject(objectName).allFields`) and are not considered.

So with `optionalFieldsAuto: all` and no explicit `optionalFields`, `getSubPages` returns `[]` and falls back to `["fields"]`, producing the empty page.

Suggested fix

In `getSubPages` (and any equivalent check), treat "has auto-discovered customer fields" as also satisfying `hasOptionalFields`:

```ts
const hasOptionalFields =
(obj.getOptionalFields("no-mappings")?.length ?? 0) > 0 ||
Object.keys(manifest.getCustomerFieldsForObject(objectName).allFields ?? {}).length > 0;
```

This way an object with only `optionalFieldsAuto: all` lands on the Additional fields sub-page once provider metadata hydrates, where `AdditionalFieldsContent` already renders the available fields as toggleable checkboxes.

Considerations

• During the loading window (before metadata hydrates), `allFields` is empty — the wizard will still briefly fall back to the empty Fields page. We should confirm whether `useSubPageNavigation` re-evaluates `getInitialSubPage` when metadata arrives, or whether we need an additional effect to re-pick the sub-page on hydration.
• Whether `optionalFieldsAuto` values other than `"all"` (if any exist) should behave the same way.

Related

Follow-up to PR #1598 which fixed the navigation case for objects with only field mappings.