Implement external_select block kit for Slack

[Franz1241]

external_select as per Slack Docs

Summary

• Adds <ExternalSelect> modal element backed by Slack's external_select block, so modals can populate dropdowns from a remote/large data source instead of being limited to static <Select> options
• Adds onOptionsLoad(actionId, handler) for registering loaders. Handlers receive {actionId, query, user, adapter, raw} and return normalized {label, value}[]. Specific action IDs run before catch-all handlers; per-handler errors are isolated
• Slack adapter now handles block_suggestion interactive payloads, calls chat.processOptionsLoad, and converts results into Slack option objects (capped at 100, per Slack's limit).
• Loader execution is capped at 2.5s to stay inside Slack's 3s response budget — on timeout the adapter still returns 200 with an empty options array and logs a warning, rather than failing the request or leaving Slack hanging.
• Documentation

Test plan

-pnpm validate runs 16/16

Closes #398

[vercel]

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[Franz1241]

Hey @dancer hope its okay to tag you. I noticed youve worked on some Slack-related PRs recently and thought you might be the right person to review this
Id appreciate it if you could take a look when you have time
This ft would help unblock my work

[dancer]

ping me anytime and will take a look later today

[Franz1241]

Tysm

[vercel]

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Project	Deployment	Actions	Updated (UTC)
chat	Ready	Preview, 💬 10 unresolved, Open in v0	Apr 21, 2026 0:47am
chat-sdk-nextjs-chat	Ready	Preview, Comment	Apr 21, 2026 0:47am

[dancer]

perfect this is great thank you so much @Franz1241 sorry for getting to it a bit late

will be on the next release cc: @bensabic

[dancer]

also ping me anytime ❤️ will make a follow up to this pr as i see 2 things e.g. initial_option & option_groups

[Franz1241]

Awesome, thanks a lot 🙌