Add Drupal config value assertion steps

[AlexSkrypnyk]

Summary

Add steps to assert (and set) a stored Drupal configuration value.

Details

ConfigOverrideTrait only disables settings.php overrides; there is no way to assert a stored Drupal configuration value. Add steps to assert that a config object's key holds an expected value, assert containment for nested/array values, and set a config value for setup. This is a frequent need when testing that a feature toggle, a third-party API endpoint, or a site setting is configured as expected, and it currently forces testers to drop out of Gherkin into custom PHP.

Proposed steps

• Assert that a config object's key equals an expected value.
• Assert that a config object's key contains an expected value (nested/array-aware).
• Set a config object's key to a value for test setup.

Acceptance criteria

• New trait, or steps in a suitable Drupal trait, under src/Drupal/.
• Reads via the Drupal config factory; writes via editable config, restored after the scenario where practical.
• Nested keys addressable with dotted notation.
• Steps use the tuple format with the Assert prefix on assertions.
• BDD coverage for equals, contains, and set.
• STEPS.md regenerated via ahoy update-docs.