fix(slack): gate feedback buttons behind rich_blocks as documented

The docs state feedback_buttons requires rich_blocks: true, but
_maybe_blocks rendered full Block Kit whenever feedback_buttons alone
was enabled — implicitly turning on rich-block rendering the user never
opted into. Align the code with the documented contract and add a
regression test.

plugins/platforms/slack/adapter.py

@@ -2040,7 +2040,7 @@ class SlackAdapter(BasePlatformAdapter):
         falls back to the plain ``text`` payload. A ``text`` fallback is ALWAYS
         sent alongside blocks, so this can safely return ``None`` at any time.
         """
-        if not self._rich_blocks_enabled() and not self._feedback_buttons_enabled():
+        if not self._rich_blocks_enabled():
             return None
         try:
             blocks = render_blocks(content, mrkdwn_fn=self.format_message)

tests/gateway/test_slack_block_kit_adapter.py

@@ -79,7 +79,7 @@ class TestSendMessageBlocks:
 
     @pytest.mark.asyncio
     async def test_feedback_buttons_opt_in_appended_to_blocks(self):
-        adapter, client = _make_adapter({"feedback_buttons": True})
+        adapter, client = _make_adapter({"rich_blocks": True, "feedback_buttons": True})
 
         await adapter.send("C1", "final answer")
 
@@ -89,6 +89,15 @@ class TestSendMessageBlocks:
         assert feedback["elements"][0]["type"] == "feedback_buttons"
         assert feedback["elements"][0]["action_id"] == "hermes_feedback"
 
+    @pytest.mark.asyncio
+    async def test_feedback_buttons_require_rich_blocks(self):
+        """feedback_buttons alone must not implicitly enable Block Kit rendering."""
+        adapter, client = _make_adapter({"feedback_buttons": True})
+
+        await adapter.send("C1", "final answer")
+
+        assert "blocks" not in client.chat_postMessage.await_args.kwargs
+
 
 class TestEditMessageBlocks:
     @pytest.mark.asyncio