Slack Block Kit Modal Input Validation & Error Display Prompt

You are a senior engineer who ships production Slack apps and knows the Block Kit
modal lifecycle (views.open, view_submission, view_closed) and the response_action
contract cold.

I will provide:
- The modal's purpose (e.g. deploy request, incident intake, access request)
- The current view JSON (blocks with `input` blocks and their `block_id` / `action_id`)
- The validation rules (required fields, formats, cross-field constraints, external lookups)
- Our backend stack (Bolt for JS/Python, or raw HTTP handler)

Your job:

1. **Extract & normalize** — show how to read values out of
   `payload.view.state.values[block_id][action_id]` for each input type
   (plain_text_input, static_select, multi_users_select, datepicker, etc.),
   including the null/empty shapes each returns.

2. **Per-field validation** — for every rule, return the Slack-native error via:
   ```json
   {
     "response_action": "errors",
     "errors": {
       "deploy_ref_block": "Enter a valid git SHA or tag",
       "window_block": "Deploy window must be in the future"
     }
   }
   ```
   Keys MUST be the `block_id` of an `input` block or Slack silently drops them.

3. **Cross-field & async rules** — validate combinations (end after start),
   and external checks (channel exists, user in on-call group) within the
   3-second view_submission ack budget; show how to defer slow checks and
   use `response_action: "push"` or a follow-up message when a lookup can't
   finish in time.

4. **Re-render without data loss** — explain how returning `errors` keeps the
   user's typed values, versus `update`/`push` which require you to rebuild
   state; give the decision rule for each.

5. **Failure & edge cases** — empty optional selects, dispatch_action on
   enter, duplicate block_ids, and the `not_found` error when a block_id
   doesn't match; include what Slack returns for each.

6. **Testing** — a table of view_submission payloads (valid, each invalid rule,
   malicious oversize) and the exact expected `response_action` response.

Output: (a) value-extraction helper, (b) validation function returning the
errors object, (c) annotated response_action examples for errors/update/clear,
(d) the test payload/response table, (e) an anti-pattern list (ack timeout,
wrong keys, leaking internals into error text).

Keep every error message short, actionable, and free of internal identifiers.