fix: replace assert with runtime guard in Slack block_kit rich_text builder

assert statements are stripped when Python runs with -O flag. Replace
the assert cur is not None in _rich_text_list_block() with an explicit
if/continue guard. The assert is technically unreachable (first loop
iteration always sets cur), but using assert for invariant checking in
production code is fragile under optimization.

plugins/platforms/slack/block_kit.py

@@ -227,7 +227,11 @@ def _list_block(items: List[Tuple[int, bool, str]]) -> Block:
             }
             elements.append(cur)
             cur_key = key
-        assert cur is not None
+        if cur is None:
+            # Defensive: should never happen (first iteration always enters
+            # the ``if key != cur_key`` block above), but guard explicitly
+            # so ``python -O`` doesn't silently drop the check.
+            continue
         cur["elements"].append(
             {"type": "rich_text_section", "elements": _nonempty_elements(_inline_elements(text))}
         )