feat(slack): convert markdown tables to Block Kit table blocks

[whatfontisthis]

Summary

When Hermes outputs markdown tables in a Slack response, the Slack adapter now converts them into Slack's supported Block Kit table blocks. Prose around tables is rendered as visible section blocks, preserving message order such as text -> table -> text and multiple tables in one response.

This replaces the earlier rich_text_table approach, which Slack rejected in live testing with invalid_blocks.

How it works

1. _parse_markdown_tables() extracts pipe-delimited markdown tables and returns ordered text/table segments.
2. _markdown_table_message_blocks() converts ordered segments into visible Slack blocks:
   • prose -> section + mrkdwn
   • table -> top-level table block
3. _tables_to_blocks() builds Slack table rows with raw_text cells, respecting Slack's table limits.
4. send() attaches blocks to chat_postMessage and falls back to plain markdown text if Slack rejects the table payload.

Behavior

• Markdown tables render as native Slack table blocks.
• Text before, between, and after tables remains visible and ordered.
• Multiple tables in one message are supported.
• If Slack returns invalid_blocks, Hermes retries as plain markdown text.
• Messages without tables are sent as before.

Tests

• ./venv/bin/python -m pytest tests/gateway/test_slack.py -k 'SendMarkdownTables' -q
• ./venv/bin/python -m py_compile gateway/platforms/slack.py

[tonydwb]

Code Review Summary

Verdict: Approved

Converts markdown tables in Slack messages to Block Kit rich_text_table blocks. Clean 1-file feature addition with proper fallback to plaintext for non-table content. Well-scoped.

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Reviewed by Hermes Agent

[teknium1]

Superseded by #56102 (#56102), a broader opt-in Slack Block Kit renderer that includes native table blocks (per-column alignment, inline-formatted cells, monospace fallback over Slack's limits) and closes #18918. This was one of several independent fixes for the same area — thanks for contributing it.