fix: Mark conversation_id as required in message schema

[markdennis22]

Why?

The conversation_id field in the message response schema is not marked as required, making it appear optional in the rendered API docs. A customer reported this was confusing — they thought there were cases where the Create Conversation API wouldn't return a conversation ID.

• https://app.intercom.com/a/inbox/tx2p130c/inbox/team/5826718/conversation/215473613867358?view=List

How?

Added conversation_id to the required array in the message schema across all API versions (v2.7–v2.10, v2.12–v2.15, and Unstable). Version 2.11 already had it and was left unchanged.

What was confusing and how this fixes it

Before	After
conversation_id rendered without a "required" badge in the response docs	conversation_id now shows the red "required" badge, matching type, id, created_at, body, and message_type
Customers thought the API might sometimes omit the conversation ID	Docs now clearly communicate that conversation_id is always returned

Companion PR in developer-docs: intercom/developer-docs#798

_{Generated with Claude Code}

[levindixon]

This adds conversation_id to the required array on the shared message schema (components/schemas/message), but that schema is referenced by two endpoints with different behavior:

Endpoint	conversation_id in response?
createConversation (POST /conversations)	Always present
createMessage (POST /messages) — admin message	Absent — stripped by messages_controller.rb:171

The spec itself already reflects this: the createMessage "admin message created" example does not include conversation_id. Marking it required in the shared schema creates a contradiction between the schema and that example.

Suggested approach: use allOf at the createConversation endpoint level only (this pattern is already used 23 times in the spec). In each version file's createConversation 200 response:

# before
              schema:
                "$ref": "#/components/schemas/message"
# after
              schema:
                allOf:
                - "$ref": "#/components/schemas/message"
                required:
                - conversation_id

Leave the shared message schema and createMessage endpoint unchanged.