[WOOMOB-54] Handle empty REST API namespaces during site login

[JorgeMucientes]

Description

Fixes WOOMOB-54

When the WordPress REST API root response contains no namespaces, site-credentials login currently treats it as a valid non-WooCommerce site and can continue into misleading fallback UI. This change treats an absent or empty namespaces field as an invalid response so the existing generic site-fetch error is shown instead. Responses with populated namespaces keep their existing behavior.

Test Steps

1. On a disposable WooCommerce test site, install and activate the Code Snippets plugin.
2. Go to Snippets > Add New, name the snippet Hide REST API namespaces, and add this code (without a <?php opening tag):

   add_filter( 'rest_index', function ( WP_REST_Response $response ) {
       $data = $response->get_data();
       $data['namespaces'] = array();
       $response->set_data( $data );
       return $response;
   } );

3. Select Run snippet everywhere, then save and activate the snippet.
4. Open https://YOUR-SITE/wp-json?_fields=namespaces and confirm the response contains "namespaces":[].
5. Start the app logged out and log in using site credentials flow (NOT WP.com)
6. Verify the app shows An error occurred while fetching your website. It must not open the Application Password authorization/tutorial flow or show the non-WooCommerce error.
7. Deactivate the snippet, repeat the login, and verify the normal site-credentials login flow resumes.

Images/gif

• I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Build Number	778
	Version	25.4-rc-1
	Application ID	com.woocommerce.android.prealpha
	Commit	d0970b1
	Installation URL	2qhhhbrrelc38

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[claude]

AI Code Review - No issues found. The changes look good.

The new guard returns an INVALID_RESPONSE error when a site's REST API exposes no namespaces, which cleanly prevents the login flow from proceeding into misleading setup screens. A couple of things I verified:

• The non-null accesses after the guard (response.name, response.namespaces.any { ... }) compile correctly because isNullOrEmpty()'s Kotlin contract smart-casts both response and response.namespaces to non-null on the fall-through path.
• The error propagates as expected: updateSite maps it via SiteErrorUtils.genericToSiteError, and WPApiSiteRepository.fetchSite logs and surfaces it as a failure.
• The behavior change is deliberate and release-noted; a site that previously "succeeded" with empty namespaces could not be verified as WooCommerce anyway, so erroring is the correct outcome.
• Test coverage is solid: empty namespaces, null namespaces, and non-empty-without-WooCommerce are all exercised.

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