M4b review, and follow-up tasks 16 to 18

The review of tasks 01 to 15 goes in the implementer log, with the
device check against the owner's Mattermost. Three follow-ups:

- 16: the refusal without a runner says why.
- 17: the texts that say who asked.
- 18: approvals from non-approvers in direct or group messages go to
  an approver.

Task 14's given serve_approvals.rs becomes serve_approvals.rs-task14,
since task 18 hands over a new version. Tip T31 is added.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M4b/files/crates/gatewayd/tests/serve_approvals.rs

@@ -112,28 +112,41 @@ fn without_a_turn_in_flight_it_goes_to_the_first_approvers_direct_messages_in_pa
     let long = format!("{{\"content\":\"{}\"}}", "x".repeat(20_000));
     let asking = ask(&socket(&home), pending(7, "chat-1789700000-1", &long)).unwrap();
     let posts = mm.posts();
-    assert_eq!(posts.len(), 2, "two parts");
+    assert_eq!(posts.len(), 3, "a first post, then two parts");
     assert_eq!(
         (posts[0].0.as_str(), posts[0].1.as_str()),
         (DM, ""),
         "a new thread in the direct channel"
     );
-    assert_eq!(
-        (posts[1].0.as_str(), posts[1].1.clone()),
-        (DM, FakeMm::post_id(0)),
-        "the next part in its thread"
-    );
     assert!(
-        posts[1]
+        posts[0]
+            .2
+            .starts_with("**Approval 7** was asked in session `chat-1789700000-1`"),
+        "{}",
+        posts[0].2
+    );
+    for (n, part) in posts[1..].iter().enumerate() {
+        assert_eq!(
+            (part.0.as_str(), part.1.clone()),
+            (DM, FakeMm::post_id(0)),
+            "part {n} in its thread"
+        );
+    }
+    assert!(posts[1].2.starts_with("**Approval 7**"), "{}", posts[1].2);
+    assert!(
+        posts[2]
             .2
             .contains("Reply in this thread with `!approve 7`")
     );
-    assert_eq!(asking.posts, [FakeMm::post_id(0), FakeMm::post_id(1)]);
+    assert_eq!(
+        asking.posts,
+        [FakeMm::post_id(0), FakeMm::post_id(1), FakeMm::post_id(2)]
+    );
     asking.close();
     let edits = mm.wait_edits(1, WAIT);
     assert_eq!(
         edits[0].0,
-        FakeMm::post_id(1),
+        FakeMm::post_id(2),
         "only the last part is edited"
     );
 }