api: Add getSingleMessage binding for GET messages/{message_id}

We'll use this for zulip#5306; see the plan in discussion:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M5306.20Follow.20.2Fnear.2F.20links.20through.20topic.20moves.2Frenames/near/1407930

In particular, we want the stream and topic for a stream message
that might not be in our data structures. We'll use this endpoint to
fetch that information.

src/api/index.js

@@ -30,6 +30,7 @@ import deleteMessage from './messages/deleteMessage';
 import deleteTopic from './messages/deleteTopic';
 import getRawMessageContent from './messages/getRawMessageContent';
 import getMessages from './messages/getMessages';
+import getSingleMessage from './messages/getSingleMessage';
 import getMessageHistory from './messages/getMessageHistory';
 import messagesFlags from './messages/messagesFlags';
 import sendMessage from './messages/sendMessage';
@@ -78,6 +79,7 @@ export {
   deleteTopic,
   getRawMessageContent,
   getMessages,
+  getSingleMessage,
   getMessageHistory,
   messagesFlags,
   sendMessage,

src/api/messages/getSingleMessage.js

@@ -0,0 +1,55 @@
+/* @flow strict-local */
+
+import type { Auth, ApiResponseSuccess } from '../transportTypes';
+import type { Message } from '../apiTypes';
+import { transformFetchedMessage, type FetchedMessage } from '../rawModelTypes';
+import { apiGet } from '../apiFetch';
+import { identityOfAuth } from '../../account/accountMisc';
+
+// The actual response from the server.  We convert the message to a proper
+// Message before returning it to application code.
+type ServerApiResponseSingleMessage = {|
+  ...$Exact<ApiResponseSuccess>,
+  -raw_content: string, // deprecated
+
+  // Until we narrow FetchedMessage into its FL 120+ form, FetchedMessage
+  // will be a bit less precise than we could be here. That's because we
+  // only get this field from servers FL 120+.
+  // TODO(server-5.0): Make this field required, and remove FL-120 comment.
+  +message?: FetchedMessage,
+|};
+
+/**
+ * See https://zulip.com/api/get-message
+ *
+ * Gives undefined if the `message` field is missing, which it will be for
+ * FL <120.
+ */
+// TODO(server-5.0): Simplify FL-120 condition in jsdoc and implementation.
+export default async (
+  auth: Auth,
+  args: {|
+    +message_id: number,
+  |},
+
+  // TODO(#4659): Don't get this from callers.
+  zulipFeatureLevel: number,
+
+  // TODO(#4659): Don't get this from callers?
+  allowEditHistory: boolean,
+): Promise<Message | void> => {
+  const { message_id } = args;
+  const response: ServerApiResponseSingleMessage = await apiGet(auth, `messages/${message_id}`, {
+    apply_markdown: true,
+  });
+
+  return (
+    response.message
+    && transformFetchedMessage<Message>(
+      response.message,
+      identityOfAuth(auth),
+      zulipFeatureLevel,
+      allowEditHistory,
+    )
+  );
+};