style: repo-wide lint pass — make the lint gate green again

Push builds have been RED on the lint step since ~build 209 from accumulated
formatting drift. This is the mechanical cleanup: ruff format + ruff --fix
(UP038 isinstance unions, SIM105 contextlib.suppress, UP031 f-strings, SIM115
tempfile context manager), shfmt -i 2 -ci, nixpkgs-fmt/statix/deadnix (merged
attrsets, dropped unused lib args), yamllint, and shell quoting fixes in
tests/lasuite-docs/setup_custom_tests.sh. No behaviour changes intended;
lint: PASS, unit tests: 138 passed.

tests/mattermost-lts/functional/test_multiuser_message.py

@@ -51,7 +51,12 @@ def test_second_user_reads_first_users_message(live_app):
     assert status in (200, 201) and team.get("id"), f"team create HTTP {status}: {team!r}"
     status, chan = harness_http.http_post(
         f"{base}/channels",
-        data={"team_id": team["id"], "name": f"c{uniq}", "display_name": f"chan {uniq}", "type": "O"},
+        data={
+            "team_id": team["id"],
+            "name": f"c{uniq}",
+            "display_name": f"chan {uniq}",
+            "type": "O",
+        },
         headers=auth_a,
         timeout=30,
     )
@@ -60,7 +65,10 @@ def test_second_user_reads_first_users_message(live_app):
     # 2) user_a posts a unique marker
     marker = f"ccci-multiuser-{uniq}"
     status, post = harness_http.http_post(
-        f"{base}/posts", data={"channel_id": chan["id"], "message": marker}, headers=auth_a, timeout=30
+        f"{base}/posts",
+        data={"channel_id": chan["id"], "message": marker},
+        headers=auth_a,
+        timeout=30,
     )
     assert status in (200, 201) and post.get("id"), f"post create HTTP {status}: {post!r}"
 
@@ -97,6 +105,6 @@ def test_second_user_reads_first_users_message(live_app):
 
     # 5) user_b sees user_a's marker (cross-user delivery, not a self read-back)
     messages = [p.get("message") for p in (posts.get("posts") or {}).values()]
-    assert marker in messages, (
-        f"user_b did not see user_a's message {marker!r} in the channel; saw {messages!r}"
-    )
+    assert (
+        marker in messages
+    ), f"user_b did not see user_a's message {marker!r} in the channel; saw {messages!r}"