[ericmann]

Ports r65272 from core trunk (#66187). It tidies the Secrets API tests and inline documentation:

• Tests follow core's PHPUnit guidelines. Each test has a summary, @covers and a failure message for every assertion, and each class has @ticket 66187. The conformance suites put @covers on the concrete classes. Multisite-only tests use the ms-required group.
• New tests cover a store that cannot list its secrets, the pinned WP_SECRETS_ERROR_* values, and a valid but different WP_SECRETS_KEY.
• In src/, the docblocks no longer refer to the feature plugin, its proposal or its decision records, and neither does the _doing_it_wrong() notice for unnamespaced names. The drop-in globals get @global annotations.

With this, src/ and the shared test helpers match trunk byte for byte again.

Beyond the straight port:

• wp_secrets_api_load_dropin() gets the same @global annotations that r65272 added to core's wp_load_secrets_dropin().
• The ported test files differ from core only where they name the plugin's drop-in loader, plus one blank line in test-wp-secrets-memzero.php that the plugin's PHPCS rules require.
• The plugin-only capabilities test is not a copy of core's, so it is unchanged.
• docs/reference/ is regenerated.

Testing

Run locally against WordPress 7.1.4:

• Single site: 505 tests, 31 skipped. Multisite: 505 tests, 20 skipped. main has 506 tests, because r65272 removes duplicated tests and adds new ones.
• PHPCS, the PHP 7.4 compatibility check and PHPStan level 10 are clean. make reference-check passes.
• The WP-CLI smoke test passes, 146 of 146.

Props peterwilsoncc.