chore(typecheck): include tsdown.config.ts in the test project

The build config was neither linted (.oxlintrc.json ignores *.config.ts) nor type-checked, so only running a build could catch a mistake in it.

tsconfig.tests.json

@@ -4,5 +4,5 @@
     // Tests import node:assert and vitest; the app code stays on types: [].
     "types": ["node"]
   },
-  "include": ["tests/host/**/*.ts", "tests/shared/**/*.ts", "tests/client/**/*.ts", "src/client/globals.d.ts", "vitest.config.ts"]
+  "include": ["tests/host/**/*.ts", "tests/shared/**/*.ts", "tests/client/**/*.ts", "src/client/globals.d.ts", "vitest.config.ts", "tsdown.config.ts"]
 }