…tifications to prevent large preview images (#31116)

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## **Description**

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Disable link unfurling in Slack RC notifications to prevent large
preview images
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## **Related issues**

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TestFlight link preview with large image displayed in the Slack RC build
notification.
Fixes: disables unfurl

## **Manual testing steps**

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- [x] I've followed [MetaMask Contributor
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#### Performance checks (if applicable)

- [x] I've tested on Android
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to import wallets with many accounts and tokens
- [x] I've instrumented key operations with Sentry traces for production
performance metrics
- See [`trace()`](/app/util/trace.ts) for usage and
[`addToken`](/app/components/Views/AddAsset/components/AddCustomToken/AddCustomToken.tsx#L274)
for an example

For performance guidelines and tooling, see the [Performance
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Single optional API flags on an internal release-notification script
with no app or security impact.
> 
> **Overview**
> RC build Slack notifications now pass **`unfurl_links: false`** and
**`unfurl_media: false`** on `chat.postMessage`, so Slack stops
expanding download, GitHub, and pipeline URLs into large link previews
in release channels.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
3448f28. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->