fix(headless): correct chat room model, message body format, and auth flow

[boiyelove]

Changes

• ChatRoomList model fix: PaginatedResponse[ChatRoomDetail] instead of PaginatedResponse[ChatRoom]. The ChatRoom wrapper always returned None for all fields because the API returns flat records, not nested under chat_room.

• create_chat_message body helper: Accepts body="plain text" and auto-converts to Circle's tiptap rich_text_body format. Previously, passing body="text" silently sent but displayed as empty/undefined in Circle.

• headless_as_user convenience method: Simplifies the two-step headless auth flow into one call: client.headless_as_user(email="...") returns a ready-to-use headless namespace.

• Return type fixes: create_chat_room, get_chat_room, update_chat_participant now return ChatRoomDetail instead of ChatRoom (both sync and async).

Before/After

# Before (broken)
room.chat_room.uuid  # AttributeError: None
create_chat_message(uuid, body="hi")  # shows empty in Circle

# After (works)
room.uuid  # "abc-123"
create_chat_message(uuid, body="hi")  # displays correctly
headless = client.headless_as_user(email="user@example.com")

Testing

Tested against live Circle.so community with 10K+ members.