`page.params.slug` throws type errors in the latest version

Describe the bug

Hi, I just updated to the latest version 2.26.1 and it started throwing errors on page.params.slug, which was not the case in the previous version 2.25.2.

As soon as I went back to the previous version, there are no errors. I see the merged PR from Rich for page.params, it's probably related to how things work now, but this can be problematic because the results are not the same as before.

Is this a breaking change?

No errors:

• "@sveltejs/kit": "^2.25.2"
• "svelte": "^5.36.15",

<!-- PageContent.svelte -->

<script lang="ts">
  import { page } from '$app/state'

  // ...

  // page.params: Page<Record<string, string>, string | null>.params: Record<string, string>
  let menuPage = $derived(getMenuPage(page.params.slug, page.url.pathname))

  // ...
</script>

Errors:

• "@sveltejs/kit": "^2.26.1"
• "svelte": "^5.36.17",

Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.ts(2345)

<!-- PageContent.svelte -->

<script lang="ts">
  import { page } from '$app/state'

  // ...

  // page.params: Page<LayoutParams<"/">, RouteId | null>.params: LayoutParams<"/">
  let menuPage = $derived(getMenuPage(page.params.slug, page.url.pathname))

  // ...
</script>

page.params.slug is now slug?: string | undefined.

Reproduction

i write a simple example above...

Logs

System Info

macos

Severity

blocking an upgrade

Additional Information

No response

Start with the PageContent.svelte reproduction and compare the inferred type of page.params.slug between @sveltejs/kit 2.25.2 and 2.26.1, using the versions and error shown in the report. Trace the merged page.params change mentioned in the issue. Done means the regression is reproduced and either the typing matches the prior behavior or the intended breaking change is clearly established.