Add migration note for token format change

Adds critical information for users about the breaking change:
- Documents automatic token migration on first access
- Advises users to regenerate URLs if they stored/bookmarked old ones
- Clarifies the version where this change occurred (v0.4.0)

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: github-actions[bot]

src/content/docs/sandbox/concepts/preview-urls.mdx

@@ -34,6 +34,10 @@ console.log(exposed.exposedAt);
 
 Preview URLs remain stable while a port is exposed and can be shared during that time. However, if you unexpose and re-expose a port, a new random token is generated and the URL changes. For persistent URLs, keep ports exposed for the duration you need them accessible.
 
+:::note[Token format change in v0.4.0]
+In SDK version 0.4.0, preview URL tokens were updated to comply with DNS hostname requirements (RFC 952/1123). Tokens now use only lowercase alphanumeric characters and hyphens instead of underscores. Existing preview URLs with underscores are automatically migrated on first access, generating new DNS-compliant tokens. If you have stored or bookmarked old preview URLs, call `exposePort()` again to get updated URLs.
+:::
+
 ## ID Case Sensitivity
 
 Preview URLs extract the sandbox ID from the hostname to route requests. Since hostnames are case-insensitive (per RFC 3986), they're always lowercased: `8080-MyProject-123.yourdomain.com` becomes `8080-myproject-123.yourdomain.com`.