fix: avoid incorrect-usage notice for missing processed-items ability

[chubes4]

Bug

The missing-ability fallback in DataMachine\Api\ProcessedItems::handle_clear() calls wp_get_ability() directly. WordPress core emits an incorrect-usage notice when the ability is not registered before returning null, so the endpoint cannot cleanly return its intended safe ability_not_found error.

Reproduction

On PR #3361 head 1c5d76757, MySQL shard 2 fails:

DataMachine\Tests\Unit\Api\ProcessedItemsEndpointTest::test_handle_clear_returns_safe_error_when_ability_is_missing
Unexpected incorrect usage notice for WP_Abilities_Registry::get_registered.

GitHub job: https://github.com/Extra-Chill/data-machine/actions/runs/32732417352/job/97452519639

The other three shards and all quality gates pass.

Expected

The endpoint should check registration through WP_Abilities_Registry::is_registered() before resolving the ability, then return the existing ability_not_found / HTTP 500 WP_Error without an incorrect-usage notice. Preserve the route, permission, input, ability execution, and response contracts.

Constraints

• Fix the missing-ability path at the owning REST adapter.
• Do not suppress _doing_it_wrong or weaken the test.
• Retain executable coverage for the clean missing-ability response.

Found during final #2424 package certification on #3361.