test: close illegal-state escape hatches in spec type-safety (WP-71)

ESLint blanket-exempted every *.spec.ts from the any ban, and no gate
type-checked spec files at all (ng test is transpile-only), so a wrong
cast in a test could never fail the build. 76 `as any` + 12 `as
Extract<>` state-narrowing casts in the three biggest wizard specs read
one variant's fields off a whole-union value: if the reducer returned
the wrong variant, the assertion silently read undefined instead of
failing.

expectTag(state, tag) (libs/shared/src/testing/expect-tag.ts) asserts
and narrows in one call, replacing every one of those casts. Removes
the spec-file any exemption, adds `npm run typecheck` (tsc --noEmit
over each project's tsconfig.spec.json) to CI, and forbids production
code from importing libs/shared/src/testing via dependency-cruiser.
Backend: AanvraagBuilder now models ZaakUrl (closing the last
post-Build() mutation) and guards AtStep; null-forgiving `!` on
endpoint assertions replaced with Assert.NotNull so a null DTO fails by
name, not NullReferenceException.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

eslint.config.mjs

@@ -48,10 +48,4 @@ export default [
     ...c,
     files: ['{apps,libs}/**/*.html'],
   })),
-
-  // Tests legitimately use `any` to feed invalid messages/states into reducers.
-  {
-    files: ['{apps,libs}/**/*.spec.ts'],
-    rules: { '@typescript-eslint/no-explicit-any': 'off' },
-  },
 ];