Fix custom tunnel URL precedence in Codespaces app dev

[afurm]

WHY are these changes introduced?

Related to #6235

When shopify app dev runs inside GitHub Codespaces, generateFrontendURL currently detects the Codespaces environment before checking --tunnel-url. That means an explicit custom tunnel URL is ignored and the CLI always builds a Codespaces forwarding URL instead.

This blocks developers who need to bring their own tunnel in Codespaces, for example to avoid embedded app preview failures caused by GitHub/Codespaces framing or cookie behavior.

WHAT is this pull request doing?

• Gives --tunnel-url precedence before Codespaces and Gitpod auto URL detection.
• Preserves the existing Codespaces and Gitpod URL behavior when no custom tunnel URL is provided.
• Adds a regression test proving custom tunnel URLs win in a Codespaces environment.
• Adds a patch changeset for @shopify/app.

How to test your changes?

Automated checks run locally:

pnpm vitest run packages/app/src/cli/services/dev/urls.test.ts
pnpm nx run app:type-check
pnpm nx run app:lint
git diff --check

Manual behavior to verify:

shopify app dev --tunnel-url="https://my-tunnel-provider.io:4242"

In a Codespaces environment, the app dev frontend URL should use https://my-tunnel-provider.io with port 4242, instead of replacing it with the Codespaces forwarding URL.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add