BDD DSL: Group non-empty YAML key assertions

[sbaum1994]

Why

Registration scenarios repeat individual non-empty assertions for generated identity fields. The file and keys are meaningful, while repeating the assertion sentence is not.

Scope

Add a table-driven assertion that named YAML keys exist and have non-empty values.

Acceptance criteria

• Add the step to the documented DSL catalog.
• Validate the table shape and reject empty keys.
• Distinguish missing keys from empty values in failures.
• Migrate existing grouped non-empty key assertions without changing coverage.
• Pass affected wiring tests and live feature tests.

Parent: #858