PURE invariant: a bridge has at most one agent-bot participant. Returns the error when existingAgentCount other IsAgent = 1 rows already exist on the bridge, else null.
existingAgentCount
IsAgent = 1
null
PURE invariant: a bridge has at most one agent-bot participant. Returns the error when
existingAgentCountotherIsAgent = 1rows already exist on the bridge, elsenull.