chore: add type checking and linting for tests

[wrslatz]

Summary

This PR implements the changes requested in #411 to add type checking and linting for test files.

Changes

• tsconfig.json: Include test directory for development with strict type checking
• tsconfig.build.json (new): Extends tsconfig.json but excludes tests and node_modules for production builds
• next.config.mjs: Configure Next.js to use tsconfig.build.json via tsconfigPath property
• eslint.config.mjs: Uncomment and enable test linting with Jest plugin configuration
• package.json: Add eslint-plugin-jest (v29.15.2) dependency
• Test file fixes: Resolved all TypeScript and ESLint errors in test files

Architecture Benefits

✅ Development: Tests included in type checking → catches bugs early
✅ Production builds: Tests excluded via tsconfig.build.json
✅ Code quality: Tests have same type safety and linting as production code
✅ Clean separation: Main tsconfig for dev/linting, tsconfig.build for builds

Testing

• ✅ All 19 tests pass
• ✅ Type checking passes (both tsconfig.json and tsconfig.build.json)
• ✅ ESLint passes (no errors in test files)
• ✅ Build succeeds

Fixes #411

[wrslatz]

Open questions:

1. Should we add TypeScript checks (tsc --no-emit) as part of CI to catch type errors not caught by the Next.js build?
2. Should we split formatting (Prettier) from linting to make it clearer which is failing, both in scripts and in CI? If others agree, I will do that in a follow-on PR.

[riley-kohler]

Addressing your open questions before I fully review:

1. Should we add TypeScript checks (tsc --no-emit) as part of CI to catch type errors not caught by the Next.js build?

I'm all for type checking being added, it's one of the main benefits of the language so we should make use of it and lift the code base up to that higher bar.

2. Should we split formatting (Prettier) from linting to make it clearer which is failing, both in scripts and in CI? If others agree, I will do that in a follow-on PR.

I'm fine with splitting them or leaving them together. It's a pretty minor change in my eyes.

[wrslatz]

Addressing your open questions before I fully review:

1. Should we add TypeScript checks (tsc --no-emit) as part of CI to catch type errors not caught by the Next.js build?

I'm all for type checking being added, it's one of the main benefits of the language so we should make use of it and lift the code base up to that higher bar.

@riley-kohler Do you think that should be added in this PR or a follow-on? And should it go in the build workflow or somewhere else?

[wrslatz]

@riley-kohler Do you think that should be added in this PR or a follow-on? And should it go in the build workflow or somewhere else?

I'll probably add a separate workflow and do it in a follow-on alongside the lint and format split change, since we'll need required statuses updated with both anyway.

[riley-kohler]

Looks good!

[riley-kohler]

All of the open question changes sound like good follow ons to me. I think type checking should just be added to the npm build script so that it always runs locally and in the build workflow. Same for lint/format, I think the CI checks should just mirror the local commands.