fix(slack): render markdown tables as Block Kit tables

[changhow]

Summary

• convert eligible Markdown pipe tables in Slack responses into native Block Kit table blocks
• preserve surrounding prose as Slack section blocks and keep existing text/mrkdwn fallback behavior
• apply the conversion to normal sends, slash-command ephemeral responses, private notices, message edits, and standalone Slack delivery
• document Slack table behavior and limits

Fixes #18918.

Note: there are older open PRs around this issue, but the active Slack implementation on main now lives under plugins/platforms/slack/adapter.py; this PR targets that current plugin path.

Behavior / safeguards

• ignores tables inside fenced code blocks
• falls back to the existing text path for invalid or over-limit tables
• enforces Slack limits: 100 rows, 20 columns, 10,000 aggregate table-cell characters per message, 50 total blocks
• uses raw_text cells after stripping Markdown delimiters that Slack table cells do not parse
• clears stale Block Kit blocks on chat.update when the edited text no longer contains a table

Tests

• python3 -m ruff check plugins/platforms/slack/adapter.py tests/gateway/test_slack.py
• git show --check --pretty=format: HEAD -- plugins/platforms/slack/adapter.py tests/gateway/test_slack.py website/docs/user-guide/messaging/slack.md
• python3 -m py_compile plugins/platforms/slack/adapter.py tests/gateway/test_slack.py
• python3 -m pytest tests/gateway/test_slack.py tests/gateway/test_send_multiple_images.py::TestSlackMultiImage -q -o 'addopts=' — 218 passed, 38 pre-existing async mock warnings

Security / review

• no new network destinations or credentials
• no shell execution or dynamic code execution
• table conversion is bounded by Slack message/table/block limits and falls back instead of sending oversized Block Kit payloads
• independent review found the multi-table aggregate cell-character cap; fixed by tracking the 10,000-character budget across all table blocks in a single message and added regression coverage

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Superseded by #56102 (#56102), a broader opt-in Slack Block Kit renderer that includes native table blocks (per-column alignment, inline-formatted cells, monospace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