test: add vercel.json config validation tests

[MaxGhenis]

Summary

Add tests that validate app/vercel.json is correctly configured for SPA routing.

What broke and why

When implementing social preview support (OG tags), I added a root vercel.json to configure API functions. This broke the entire site with 404 errors on all routes because:

1. Root vercel.json overrides app/vercel.json - Vercel uses the config file closest to the project root, so my new root config took precedence over the working app/vercel.json

2. Wrong routing config - I tried various approaches (rewrites, routes with handle: filesystem) but none worked correctly because:

   • The original working config used { "source": "/(.*)", "destination": "/" } in app/vercel.json
   • My root config used different patterns and paths that didn't match the outputDirectory setup

3. SPA routing requires specific config - For React Router to handle client-side routes, all requests must be rewritten to serve index.html. Without this, direct URL access (e.g., refreshing /uk/research/some-article) returns 404 because Vercel looks for a literal file at that path.

Timeline of the incident

Commit/PR	Description	Impact
1113ee30	Added root vercel.json to enable API functions	🔴 Broke all routes with 404
0db14439	Attempted fix: changed rewrite to serve index.html	🔴 Still broken
#437	Attempted fix: switched to routes with filesystem handler	🔴 Still broken
#438	Fix: Removed root vercel.json, restored app/vercel.json	✅ Site working

What these tests catch

These tests would have immediately failed when the root vercel.json was added:

✗ given root vercel.json then does NOT exist (app/vercel.json is canonical)

And would have caught incorrect rewrite configurations:

✗ given vercel.json rewrites then has SPA fallback rule
  Expected: { source: "/(.*)", destination: "/" }

Tests added

1. Verifies app/vercel.json exists
2. Verifies it contains valid JSON
3. Verifies it has rewrites array
4. Verifies it has the SPA fallback rule { source: "/(.*)", destination: "/" }
5. Verifies no routes array (which breaks SPA routing when combined with rewrites)
6. Verifies no root vercel.json exists (which caused 404 errors)

Test plan

• Tests pass locally
• CI passes

🤖 Generated with Claude Code

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
policyengine-app-v2	Ready	Preview	Comment	Nov 25, 2025 11:14am

[anth-volk]

Will review today

[anth-volk]

Approved. Going to break the code freeze rule, since this PR is tests only.