Join two entities

Creates one directed relationship between two existing entities and returns the row. The created row is always isCurrent=true and isPrimary=true, so it appears in default relationship reads. relationshipType must be joinable=true from entities relationships types; its direction/sourceRole/targetRole fields describe whether source/target order is symmetric, auto-oriented, or directional. For parent, sourceEntityId is the parent and targetEntityId is the child. For affinity, sourceEntityId is the member organization and targetEntityId is the provider organization; the companies list affinity filter accepts provider names only. For productService, one side must be Product or Service and the provider side must be Company, Nonprofit, Government, or Business Line; Investment Firm, Fund, and group-only Organization are rejected because they cannot own products/services. AcquisitionManaged and readOnly types are rejected. For acceleratorParticipant, one side must be the participant entity and the other side must be the accelerator entity with typeRecord=Investment Firm; the join endpoint stores participant as source and accelerator as target. acceleratorParticipant requires detail as `batch=<label>` or `program=<name>; batch=<label>` plus asOf. Use entities relationships row create when writing historic/non-primary curation flags. Missing or malformed query parameters return 400. A relationshipType whose source/target entity-type pairing is not accepted, an acquisitionManaged or readOnly relationshipType, missing acceleratorParticipant detail/asOf, or sourceEntityId equal to targetEntityId returns 422. A productService join that would create an ambiguous Product/Service slug route, a duplicate relationship, or parent/acquisition exclusivity for the same unordered entity pair returns 409; duplicate directional conflicts name the existing row and the required sourceRole/targetRole orientation. An unresolved sourceEntityId or targetEntityId returns 404.