cf-removal: add v0.10.0 migration + tests

New class PRAutoBlogger_Migrate_Remove_Cloudflare_V0100 (own file so
class-activator.php stays under 300 lines). Runs from
PRAutoBlogger::maybe_run_db_migrations() right after the v0.9.0
migration. Idempotent via prautoblogger_migrated_remove_cloudflare_v0100
flag option (picked up by the uninstall prefix sweep).

Migrates any user on a Cloudflare image provider to Runware FLUX.1
schnell (closest equivalent, ~bash.0006/image). Detects the three
shapes a legacy user can be in: registry-derived provider ===
'cloudflare', image_provider option === 'cloudflare', or the legacy
slug 'flux-1-schnell'.

Cleans up three now-dead options: prautoblogger_cloudflare_ai_token,
prautoblogger_cloudflare_account_id, prautoblogger_cf_image_via_gateway.
Does NOT touch prautoblogger_ai_gateway_base_url — that option is
shared with the OpenRouter AI Gateway route and must stay (live-
verified test).

Tests (tests/unit/Core/ActivatorRemoveCloudflareTest.php):
- cloudflare user → runware schnell
- non-cloudflare user preserved
- runware user preserved
- legacy 'flux-1-schnell' slug triggers migration
- second run is a no-op (does not clobber admin manual changes)
- AI Gateway URL preserved across every path

ImageModelRegistryTest updated: allowed-provider set is now
['runware','openrouter']; new test_cloudflare_entries_removed_v0100
test pins the scope boundary.

includes/core/class-image-pipeline.php

@@ -54,12 +54,8 @@ class PRAutoBlogger_Image_Pipeline {
 	}
 
 	/**
-	 * Instantiate the image provider based on the admin setting.
-	 *
-	 * Returns Runware by default when the setting is unrecognised —
-	 * Cloudflare Workers AI was removed as a provider in v0.10.0; legacy
-	 * 'cloudflare' values are migrated at admin_init, but an unmigrated
-	 * install should still get a working fallback rather than a fatal.
+	 * Instantiate the image provider based on the admin setting. Defaults
+	 * to Runware on unrecognised values (CF was removed in v0.10.0).
 	 *
 	 * @return PRAutoBlogger_Image_Provider_Interface
 	 */