test(telegram): cover bot self-message ingestion guard (#11905)

Regression tests for the self-author guard added in the salvaged fix:
- bot-authored DM-topic watcher echo is dropped (the exact #11905 symptom)
- bot self-messages dropped in groups/supergroups too
- other bots in the same chat are still processed (self-id, not is_bot)
- observe-unmentioned sibling path also rejects self-messages
- missing from_user does not crash

Test scaffolding ported from @cola-runner's PR #12817 and adapted to the
current plugins/platforms/telegram/adapter.py and _is_own_message().

tests/gateway/test_telegram_group_gating.py

@@ -636,6 +636,66 @@ def test_invalid_regex_patterns_are_ignored():
     assert adapter._should_process_message(_group_message("hello everyone")) is False
 
 
+def test_bot_self_messages_are_ignored_in_dm_and_group():
+    """Bot-authored messages must not re-enter as fresh user turns (issue #11905).
+
+    Telegram echoes the bot's own outbound messages back through getUpdates.
+    Without a self-author guard, those echoes — including
+    ``[SYSTEM: Background process ...]`` watcher notifications — get ingested
+    as new inbound turns, producing the "haunted topic" loop. The guard keys
+    on ``from_user.id == self._bot.id`` (bot id is 999 in ``_make_adapter``).
+    """
+    adapter = _make_adapter(require_mention=False)
+
+    # Control: a real user in the same group IS processed.
+    assert adapter._should_process_message(_group_message("hi", chat_id=-100)) is True
+
+    # The exact reported symptom: a bot-authored DM-topic watcher echo.
+    self_dm = _group_message(
+        "[SYSTEM: Background process matched watch pattern ...]",
+        chat_id=555,
+        from_user_id=999,
+    )
+    self_dm.chat.type = "private"
+    assert adapter._should_process_message(self_dm) is False
+
+    # Same guard applies in groups/supergroups.
+    self_group = _group_message("status tick", chat_id=-100, from_user_id=999)
+    assert adapter._should_process_message(self_group) is False
+
+
+def test_other_bots_are_still_processed():
+    """A different bot's message must not be over-filtered.
+
+    Distinguishes the self-id guard from a blanket ``from_user.is_bot`` check,
+    which would incorrectly drop unrelated bots (weather, music, etc.) sharing
+    the same chat.
+    """
+    adapter = _make_adapter(require_mention=False)
+    other_bot = _group_message("weather update", chat_id=-100, from_user_id=555)
+    other_bot.from_user = SimpleNamespace(id=555, is_bot=True)
+    assert adapter._should_process_message(other_bot) is True
+
+
+def test_self_message_guard_skips_observe_path():
+    """Bot-authored messages are not stored via the observe-unmentioned path.
+
+    When ``_should_process_message`` rejects a message, dispatch falls through
+    to ``_should_observe_unmentioned_group_message``; the self-guard must also
+    sit there so a self-echo is neither dispatched nor stored.
+    """
+    adapter = _make_adapter(require_mention=True, observe_unmentioned_group_messages=True)
+    self_group = _group_message("status tick", chat_id=-100, from_user_id=999)
+    assert adapter._should_observe_unmentioned_group_message(self_group) is False
+
+
+def test_missing_from_user_does_not_crash():
+    adapter = _make_adapter(require_mention=False)
+    anon = _group_message("channel post", chat_id=-100)
+    anon.from_user = None
+    assert adapter._should_process_message(anon) is True
+
+
 def test_config_bridges_telegram_group_settings(monkeypatch, tmp_path):
     hermes_home = tmp_path / ".hermes"
     hermes_home.mkdir()