Creates a new segment configuration.

Provide name, action, and the ordered segments[] (each with its outputValue and rules[]). mergeOperator is optional and defaults to ,. Server-assigned fields (id, per-segment order, fieldsUsedInRules, and timestamps) are ignored on input and returned in the response.

Returns 201 with the created segment config. Returns 400 if the payload is malformed (e.g. unknown action, a rule missing its field, or a segment missing its outputValue).