test: reproduce resolve failure with RouteId unions

[teemingc]

closes #15536

Adds an end-to-end type regression test demonstrating that a generated RouteId union cannot be passed to resolve(). The empty static routes intentionally exceed TypeScript's inference cutoff, which is required to expose the full-union error.

This PR is expected to fail until the implementation fix is added separately.

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from b65ac8e:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/b65ac8e75617fd175d9b5cc7a4566067c5c3f379

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17024

[changeset-bot]

⚠️ No Changeset found

Latest commit: b65ac8e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-17024-svelte.vercel.app/