Answer a schema write with the change set it joined #2615

Part of #2423 (F-MCP). A follow-up from the review of #2520 (SCH-05).

Since #2520, create_content_type and revise_content_type let the content type directory record the operation. The tool answers with the change set by reading it again after the write commits, and for a session_id it reads the session's open set. Between the write and that read, the same agent can submit the set, or a concurrent write can open the session's next one:

• If the set was submitted, a write that committed is answered with change_set_not_found. The agent may then retry and store a second draft of the same change.
• If the next set was opened, ProposedWrite does not find the item's operation in it and throws.

Scope

• The content type write tells its caller which change set it joined, so the tool answers from that set rather than from a second lookup by session.
• The answer reads the set by its id. A set submitted in the meantime still answers with the write's operation and the set's new state.

Out of scope

• Entry writes, which add their operation through IChangeSetDirectory and already answer with the set it returns.

Acceptance

• A schema write whose session set is submitted before the tool reads it back answers with the write's version and that set's id.
• A schema write whose session's next set was opened concurrently answers with the set it joined and does not throw.
• All of the above passes on SQLite and PostgreSQL.