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)