test(rest-api): improve REST API trait tests

[superdav42]

Task t472

File: inc/apis/trait-rest-api.php
Current coverage: 31.6%
Uncovered statements: 160
Estimated time: ~3h

Improve REST API trait test coverage to ≥80%.

Acceptance Criteria

• trait-rest-api.php coverage reaches ≥80%
• All new tests pass in CI
• No regressions in existing tests

Context

Part of the unit test coverage improvement initiative. See TODO.md t472.

[superdav42]

What was done

• Added tests/WP_Ultimo/Apis/Rest_Api_Trait_Test.php with 60 tests covering all methods in inc/apis/trait-rest-api.php
• Implemented stub model and manager classes to exercise the trait in isolation without real DB dependencies
• All 60 tests pass across PHP 8.2, 8.3, 8.4, and 8.5 in CI

Testing Evidence

• Testing level: unit-tested
• Risk classification: low (test files only, no production code changes)
• Local result: OK (60 tests, 110 assertions)
• CI result: PHP 8.2 ✅, PHP 8.3 ✅, PHP 8.4 ✅, PHP 8.5 ✅

Key decisions

• Used stub model/manager classes rather than real WP_Ultimo models to avoid DB dependencies and keep tests fast
• Used setExpectedIncorrectUsage('register_rest_route') to suppress WP test framework notices when calling route registration outside rest_api_init
• Used extract_wp_error() helper to handle both WP_Error and WP_REST_Response(WP_Error) return patterns across WP versions

Files changed

• tests/WP_Ultimo/Apis/Rest_Api_Trait_Test.php — new test file, 1353 lines, 60 tests

Blockers

None

Follow-up needs

None — coverage target met

Released in

PR #578 (merged)