fix(slack): harden rich table block fallback

[hashbender]

Summary

Fixes a live Slack API regression from the merged NousResearch#56102 rich_blocks table path.

This keeps the selected Block Kit renderer architecture intact and makes a narrow schema/fallback hardening change:

• emit only valid object entries in table column_settings, never null
• omit trailing default-left column_settings entries so Slack applies defaults
• retry once without blocks when Slack rejects a rich Block Kit payload as invalid/too large
• add regressions for Slack-valid table payloads and no-block retry behavior

Fixes NousResearch#56615

cc @teknium1, this is the focused follow-up from the live validation comment on NousResearch#56102.

Validation

scripts/run_tests.sh tests/gateway/test_slack.py tests/gateway/test_slack_block_kit.py tests/gateway/test_slack_block_kit_adapter.py -q
# 247 tests passed, 0 failed

ruff check plugins/platforms/slack/adapter.py plugins/platforms/slack/block_kit.py tests/gateway/test_slack_block_kit.py tests/gateway/test_slack_block_kit_adapter.py
# All checks passed

git diff --check
# clean

Notes

This intentionally does not port the broader NousResearch#53893 hardening set. Those can be separate follow-ups. This PR only fixes the current merged table-block regression and prevents rich formatting failures from dropping Slack replies.

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Mirror-of: NousResearch#56618
NousResearch#56618

[tenki-reviewer]

Review Complete
No issues found!

Risk: 🟢 Low (15/100) — no findings · 214 LOC across 4 files

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PR adds retry logic to Slack adapter for block-payload rejections and refines Block Kit column_settings rendering. One low-confidence finding was identified below threshold (dead error code in block-rejection detection targeting wrong Slack API surface).

Files Reviewed (4 files)

plugins/platforms/slack/adapter.py
plugins/platforms/slack/block_kit.py
tests/gateway/test_slack_block_kit.py
tests/gateway/test_slack_block_kit_adapter.py