Deliver a frontend tool execution result.

Deliver the result of a frontend tool execution from the customer's app. Called by the Gleap SDK. Requires the project API key in the api-token header plus a valid widget session (gleap-id/gleap-hash headers); returns 401 otherwise and 403 if the calling domain is not an allowed widget domain. result/error payloads above 50,000 characters are rejected with 400; accepted payloads are truncated to 20,000 characters before being handed to the agent. (Gleap.registerAgentTool handler) back to the awaiting agent.