LegacyUserAgent.post(url: string,kind: "form" | "json",data: Record<string, string> | Record<string, unknown>): Promise<{ result: LegacyHttpResult; }>
kind mirrors Mojo::UserAgent's own ->post($url => form => {...}) /
->post($url => json => {...}) calling convention — form URL-encodes data as
application/x-www-form-urlencoded (string values only, matching real HTML form semantics);
json sends it as a application/json body (data may be any JSON-serializable value —
verified against the one real json-kind call site in the corpus, which passes a nested
object with an array value, not just flat strings).
Promise<{ result: LegacyHttpResult; }>