docs(api): drop dry_run from member notify, document html validation 400

[ysyneu]

Summary

• Remove dry_run from MemberNotifyRequest and html from MemberNotifyResponse in all four OpenAPI spec files (openapi.{en,zh}.json, platform.openapi.{en,zh}.json).
• Document the new mandatory server-side validation: the raw html is inspected before sanitizing, and a construct whose removal would change what recipients see (<style>, <svg>, <iframe>, <object>, <embed>, <form>, <input>, <button> tags; an <img> with a missing or non-https src; a link outside http/https/mailto) now returns 400 / InvalidParameter instead of being silently stripped. Constructs whose removal never changes rendering (<script>, <meta>, <link>, <base> tags, on* handlers) are still sanitized silently.
• Update the Go SDK reference page (developer/go-sdk.mdx, en/zh), which documented the same removed field.

dry_run made "does this call have a side effect" a boolean inside the request body, while the documented workflow builds one body and reuses it for both calls. The response echoed no mode, so the only discriminator was whether html came back non-empty — and the generated SDK declares html without omitempty, so a real send also returns the key. The result was a call that could silently do nothing while looking exactly like a successful send. Replacing the optional pre-flight with a mandatory server-side check removes the mode instead of documenting it.

Test plan

• python3 -m json.tool on all four spec files — valid JSON
• git diff --stat — touches only the member-notify schema/path areas plus the two go-sdk.mdx paragraphs
• git grep dry_run across the repo — no remaining references