Inline related resources on a single PayeeRequirement read. Supported value is Configuration — a redacted summary of the resource that configurationId addresses (the requirement DEFINITION's own backing template/configuration — distinct from dataSourceId, this INSTANCE's own backing resource). Unsupported values return 422 instead of being silently ignored. Expansions are not available on list rows.
The summary is a deliberate SUBSET of the full configuration resource: payer-only fields (template-editing links, owner/viewer lists, the payer's own annotations) are never surfaced here. Requesting Configuration is never an error: the key is simply omitted when there is nothing to inline — a requirement type with no configuration, a null configurationId, or a lookup that cannot resolve one. A supported value that resolves nothing is a 200 with the key absent; only an unsupported value is a 422.
