Render Slack progress-mode updates as native task-card progress blocks, with bounded Slack chunk text and stable fallback behavior.
Also deep-merge Slack account streaming objects over top-level defaults while preserving legacy scalar account overrides, and keep the plugin SDK fetch runtime import path from evaluating guarded-fetch dispatcher code.
Verification:
- pnpm test extensions/slack/src/progress-blocks.test.ts extensions/slack/src/accounts.test.ts src/plugin-sdk/fetch-runtime.test.ts
- pnpm lint --threads=8
- git diff --check
- .agents/skills/autoreview/scripts/autoreview --mode local
- GitHub PR checks green on #87748 at 4803e98820
Refs #82258
Co-authored-by: Simon van Laak <32648751+simonvanlaak@users.noreply.github.com>
