tool_req__docs_saf_attrs_mitigated_by Make mitigated_by mandatory for valid safety analysis needs

### What

Make mitigated_by mandatory for valid safety analysis needs

### Acceptance Criteria (DoD)

Make mitigated_by mandatory for valid safety analysis needs done

### How

_No response_

No file, test, or entry point is identified; start by locating the definition or validation for safety analysis attributes and the `mitigated_by` field. Done means valid safety analysis needs require `mitigated_by`, with coverage in the relevant validation or documentation tests if present.

Written by the indexing model from the issue text.