Gate taxonomies REST API migration behind a remote feature flag

[adalpari]

Description

The post categories taxonomy was migrated from XML-RPC to the REST API. For self-hosted sites with an Application Password, TaxonomyStore routes to the new wp-rs REST client (TaxonomyRsApiRestClient); previously this was gated only on site.isUsingSelfHostedRestApi().

This PR adds a new server-side remote feature flag, taxonomies_rest_api_migration, so the new self-hosted path can be rolled out (or reverted) remotely. The wp-rs path is now used only when both the Application-Password check passes and the flag is enabled. When the flag is off, self-hosted sites fall back to the exact pre-migration behavior (XML-RPC).

Note: this flag gates the whole self-hosted taxonomy path in TaxonomyStore, which covers both categories and tags — hence the taxonomies_ prefix.

Changes:

• New taxonomies_rest_api_migration feature flag (TaxonomiesRestApiMigrationFeatureConfig, default off), wired via a small fluxc-side TaxonomiesRestApiMigrationConfig interface that the app implements in AppConfigModule (same app→fluxc bridge pattern used for UserAgent/AppSecrets, since fluxc can't depend on the app module).
• TaxonomyStore now gates all self-hosted wp-rs routing through a single shouldUseTaxonomyRsApiRestClient(site) helper: fetchTerms, pushTerm, and the response-already-contains-term branch of handlePushTermCompleted (so an off-flag push correctly takes the XML-RPC follow-up-fetch path).
• deleteTerm is also migrated to the wp-rs client under the same flag, closing a prior gap where self-hosted deletes always went to XML-RPC.

Note on the singular fetchTerm path: it is intentionally left ungated. The FETCH_TERM action (which routes to the single-term fetchTerm handler) has no dispatchers anywhere in the codebase — a repo-wide search for newFetchTermAction returns zero callers — so it was never migrated to the wp-rs client and is unreachable in production. The only reachable self-hosted taxonomy operations are fetchTerms (list), pushTerm, and deleteTerm, all of which this PR gates. (The mTaxonomyXMLRPCClient.fetchTerm(...) call inside handlePushTermCompleted is a direct call on the XML-RPC follow-up branch, not the gated path, so it stays correct by construction.)

Testing instructions

Routing logic is covered by unit tests in TaxonomyStoreRoutingTest (flag on → wp-rs client, flag off → XML-RPC, for fetch/push/delete; plus wp.com unaffected). Run them with:

./gradlew :libs:fluxc:testDebugUnitTest --tests "org.wordpress.android.fluxc.taxonomy.TaxonomyStoreRoutingTest"

Manual verification on a self-hosted (Application Password) site:

You can turn ON(OFF the FF by going to Me -> Debug Settings

Flag OFF (default):

1. Open a self-hosted site and edit a post's categories; add and edit a category.

• Verify via AppLog.T.API logs that XML-RPC endpoints are used and create/edit still resolves the slug

2. Delete a category.

• Verify the delete goes through XML-RPC

Flag ON (enable taxonomies_rest_api_migration remotely or via a debug override):
3. Repeat fetch / create / edit / delete of categories.

• Verify wp-rs REST endpoints are used and operations complete with no XML-RPC follow-up fetch

WordPress.com / Jetpack sites:
4. Manage categories on a WP.com site with the flag both on and off.

• Verify behavior is unchanged (still uses the WP.com REST client)

[dangermattic]

	2 Warnings
⚠️	Class TaxonomiesRestApiMigrationFeatureConfig is missing tests, but unit-tests-exemption label was set to ignore this.
⚠️	PR is not assigned to a milestone.

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[wpmobilebot]

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

	App Name	WordPress Android
	Build Type	Debug
	Version	pr23029-ab9b1c6
	Build Number	1497
	Application ID	org.wordpress.android.prealpha
	Commit	ab9b1c6
	Installation URL	1hgoudbg5cup0

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[wpmobilebot]

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

	App Name	Jetpack Android
	Build Type	Debug
	Version	pr23029-ab9b1c6
	Build Number	1497
	Application ID	com.jetpack.android.prealpha
	Commit	ab9b1c6
	Installation URL	5e7vkhu9416go

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

[wpmobilebot]

🤖 Build Failure Analysis

This build has failures. Claude has analyzed them - check the build annotations for details.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.57%. Comparing base (43849a5) to head (ab9b1c6).

Files with missing lines	Patch %	Lines
...g/wordpress/android/fluxc/store/TaxonomyStore.java	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release/26.9   #23029      +/-   ##
================================================
+ Coverage         37.56%   37.57%   +0.01%     
================================================
  Files              2325     2326       +1     
  Lines            125324   125332       +8     
  Branches          17187    17189       +2     
================================================
+ Hits              47073    47094      +21     
+ Misses            74433    74414      -19     
- Partials           3818     3824       +6     

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[nbradbury]

Looks good!