v0.4.26.3 — Slack render_postable AST fix

Python-only patch on top of vercel/chat@4.26.0. No upstream version change.

Fixes

• SlackFormatConverter.render_postable now uses the AST path for all markdown inputs (#81). Previously, PostableMarkdown and {"markdown": ...} dict inputs were routed through a private regex helper (_markdown_to_mrkdwn) that truncated URLs containing parentheses and diverged silently from the TS SDK's fromAst(parseMarkdown(text)) behavior. Both branches now call from_markdown, which goes through the AST. str and raw branches are unchanged.

Structural parity

• Deleted _markdown_to_mrkdwn — a regex-based private method with no call sites after the fix above. The TS SDK has no equivalent; its presence was an undocumented divergence.

Additions

• render_postable now handles card and object-with-ast inputs — added {"card": ...} dict, {"type": "card", ...} CardElement dict, {"ast": ...} dict, and .card / .ast attribute branches, plus str(message) fallback for unrecognized types. Matches the full union of AdapterPostableMessage variants.

Test quality

19 new tests in tests/test_slack_format.py (47 → 66) covering all render_postable branches, every _node_to_mrkdwn node type (heading, blockquote, thematic break, image with/without alt), the remaining extract_plain_text paths (strikethrough, bare URL, channel mentions), and to_blocks_with_table edge cases (non-dict AST, standalone table, column alignment).

Closes: #81 · PR: #82