feat(slack): use markdown block for native table rendering

[gonzalofrancoceballos]

Replaces legacy mrkdwn text path with Slack's markdown block type (April 2026), which natively renders standard markdown including tables, bold, links, code blocks, lists, and headers.

Fixes #26947. Related: #8552.

Changes

gateway/platforms/slack.py

• send(): Wraps content in {"type": "markdown", "text": content} block. format_message() output as text fallback. >12K chars falls back to legacy mrkdwn path.
• edit_message(): Same markdown block treatment for streaming edits.

tools/send_message_tool.py

• _send_slack(): Uses markdown blocks instead of mrkdwn: True.
• _send_to_platform(): No longer calls format_message() for Slack — raw markdown must be preserved for the markdown block.

[alt-glitch]

Duplicate of #8554 (and previously closed #35133 by the same author). Also overlaps with #19108. All implement the same Slack Block Kit markdown block migration for #8552.

[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Superseded by #56102 (#56102), a broader opt-in Slack Block Kit renderer that includes native table blocks (per-column alignment, inline-formatted cells, monospace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.