Closes #1549.

Added the new blocks as announced in
https://docs.slack.dev/changelog/2026/04/16/block-kit-new-blocks except
for `Alert` which doesn't yet seem to work (not in any of my
environments at least).

`Alert` will come later in a different PR once I can test in a sandbox
environment and confirm it working.

Slack's
[assistant.threads.setStatus](https://docs.slack.dev/reference/methods/assistant.threads.setStatus)
added a couple of additional optional parameters: `username`,
`icon_url`, and `icon_emoji` for customization. This PR adds these new
parameters to `SetAssistantThreadsStatusContext`.

Changes:
- updates `AssistantThreadsSetStatusParameters` to include these new
parameters
- updates `SetAssistantThreadsStatusContext` to forward these new
parameters when set
- updates existing test to include these fields

| icon_url | icon_emoji |
| --- | --- |
|<img width="147" height="217" alt="Screenshot 2026-04-20 at 12 36
03 PM"
src="https://github.com/user-attachments/assets/a4a096f6-276e-4eb7-bef7-ddbb0879ba65"
/>|<img width="185" height="213" alt="Screenshot 2026-04-20 at 1 00
10 PM"
src="https://github.com/user-attachments/assets/fe37a440-6b45-4fa7-84b8-defa459c704c"
/>|

Adds `AlertBlock`, the third agent-UI block from the April 16 Slack
changelog. Uses the functional-options pattern (`NewAlertBlock`) with
severity via `AlertBlockOptionLevel`
(default/info/warning/error/success) and block ID via
`AlertBlockOptionBlockID`. Wires into `Blocks.UnmarshalJSON` for
round-trip fidelity and adds coverage in `block_alert_test.go`.

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>