feat(api): filter a conversation's messages by kind

A chat reopened while its flow is still running has to find the message
that started the turn: it carries the flow job, which is the only thing
that knows whether the run is over. An agent writes a row per round and
per tool call, so that message sits an unbounded number of pages back in
the transcript, and a client reading towards it is searching with no end
it can name.

`message_type` answers it in one request. The filter is applied inside
the paginated subquery, so `page=1&per_page=1&message_type=user` really
is the newest user row rather than the newest row that happens to be one.

`MessageType` carries a fourth kind, `System`, that nothing writes and
the Postgres enum has no label for. A request to filter on it is refused
before the database, so what reaches the caller is a word about their
request rather than an error about an enum.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/windmill-api/openapi.yaml

@@ -12507,6 +12507,16 @@ paths:
           schema:
             type: integer
             format: int64
+        - name: message_type
+          description: Keep only the messages of that kind
+          in: query
+          required: false
+          schema:
+            type: string
+            # The labels the MESSAGE_TYPE column actually has. The response schema below
+            # carries a fourth, `system`, which nothing writes and the column cannot hold;
+            # the handler refuses a request to filter on it.
+            enum: [user, assistant, tool]
       responses:
         "200":
           description: conversation messages