test(slack): give the MPIM reaction-guard test real teeth

The reaction-guard regression test defined a local _should_react lambda and
asserted it against itself — a tautology that would stay green even if the
production guard at _handle_slack_message reverted to (is_dm or is_mentioned),
re-introducing the unmentioned-MPIM reaction spam this PR fixes.

Replace it with a shared _reaction_guard helper plus a source-introspection
test that pins the production expression: asserts (is_one_to_one_dm or
is_mentioned) is present and (is_dm or is_mentioned) is absent. Mutation-checked
— reverting the adapter guard now fails the test.

Follow-up self-review finding on the salvage of #57339.

tests/gateway/test_slack_mention.py

@@ -5,6 +5,7 @@ Follows the same pattern as test_whatsapp_group_gating.py.
 """
 
 import sys
+import inspect
 from unittest.mock import MagicMock
 
 from gateway.config import Platform, PlatformConfig
@@ -324,6 +325,19 @@ def test_dm_always_processed_regardless_of_setting():
 # mpim as a DM and thereby exempted it from mention gating + reaction guards).
 # ---------------------------------------------------------------------------
 
+def _reaction_guard(channel_type, is_mentioned):
+    """Mirror of the production reaction guard in ``_handle_slack_message``:
+
+        _should_react = (is_one_to_one_dm or is_mentioned) and reactions_enabled
+
+    Only a true 1:1 IM or an explicit @mention earns a reaction; MPIMs and
+    channels must be @mentioned. ``test_reaction_guard_pinned_to_production_expression``
+    pins this to the real source so the two cannot silently drift.
+    """
+    is_one_to_one_dm = channel_type == "im"
+    return is_one_to_one_dm or is_mentioned
+
+
 def test_mpim_unmentioned_strict_mention_ignored():
     """MPIM, not mentioned, strict_mention on -> dropped (shared surface)."""
     adapter = _make_adapter(require_mention=True, strict_mention=True,
@@ -369,14 +383,29 @@ def test_one_to_one_im_still_exempt():
 def test_mpim_unmentioned_does_not_react():
     """Reaction guard: only a 1:1 IM or an @mention earns a reaction. An
     unmentioned MPIM message must NOT get :eyes:/:white_check_mark: noise."""
-    def _should_react(channel_type, is_mentioned):
-        is_one_to_one_dm = channel_type == "im"
-        return is_one_to_one_dm or is_mentioned
+    assert _reaction_guard("mpim", False) is False   # the reported spam case
+    assert _reaction_guard("mpim", True) is True      # addressed -> ok
+    assert _reaction_guard("im", False) is True        # 1:1 DM -> ok
+    assert _reaction_guard("", False) is False         # channel, unmentioned
 
-    assert _should_react("mpim", False) is False   # the reported spam case
-    assert _should_react("mpim", True) is True      # addressed -> ok
-    assert _should_react("im", False) is True        # 1:1 DM -> ok
-    assert _should_react("", False) is False         # channel, unmentioned
+
+def test_reaction_guard_pinned_to_production_expression():
+    """Regression teeth for the reaction guard.
+
+    ``_reaction_guard`` mirrors the production expression at the
+    ``_should_react = (is_one_to_one_dm or is_mentioned) ...`` site in
+    ``adapter.py``. This test pins that source line so a revert of the fix
+    (back to ``is_dm or is_mentioned``, which reacts to unmentioned MPIMs)
+    fails here instead of silently passing a self-referential lambda.
+    """
+    src = inspect.getsource(SlackAdapter._handle_slack_message)
+    assert "(is_one_to_one_dm or is_mentioned)" in src, (
+        "reaction guard no longer keys off is_one_to_one_dm — an unmentioned "
+        "MPIM would react again (regression of the group-DM fix)"
+    )
+    assert "(is_dm or is_mentioned)" not in src, (
+        "reaction guard reverted to is_dm — MPIMs would react when unmentioned"
+    )
 
 
 def test_mentioned_message_always_processed():