fix(slack): keep blank-line-separated ordered items in one rich_text_list

When a Markdown ordered list has blank lines between items (common in
LLM-authored content), the list run loop breaks on each blank line.
Slack numbers each rich_text_list independently, so N items produce N
lists each starting at 1.

Skip blank lines inside the list run as soft separators instead of
breaking, so ordered items stay in one rich_text_list and Slack renders
the correct numbering.

Fixes #57076

plugins/platforms/slack/block_kit.py

@@ -451,6 +451,10 @@ def render_blocks(
                         indent, ordered, txt = items[-1]
                         items[-1] = (indent, ordered, txt + " " + lines[i].strip())
                         i += 1
+                    elif not lines[i].strip():
+                        # blank line — soft separator within a list run;
+                        # skip so that ordered items stay in one rich_text_list.
+                        i += 1
                     else:
                         break
                 blocks.append(_list_block(items))