fix(structures): widen via String() in narrowing test to satisfy lint (#231)

## Summary

One-line lint fix in [apps/portal-bff/src/structures/structure-kind.spec.ts](apps/portal-bff/src/structures/structure-kind.spec.ts) — the narrowing test failed CI lint with `@typescript-eslint/no-inferrable-types` on `const candidate: string = 'medico_social'`.

The naive fix (drop the annotation) would make the test vacuous: TypeScript would infer the literal type `'medico_social'`, which is already a `StructureKind` subtype, so the runtime guard `isStructureKind` would have nothing to prove. Fix instead: widen via `String('medico_social')` — the inferred return type is `string`, the narrowing check stays meaningful, the linter is happy.

## What lands

| File | Change |
| --- | --- |
| `apps/portal-bff/src/structures/structure-kind.spec.ts` | `const candidate: string = 'medico_social'` → `const candidate = String('medico_social')`. 3-line comment explains why `String()` rather than dropping the annotation. |

## Test plan

- [x] `pnpm exec nx lint portal-bff` — `0 errors, 13 warnings` (warnings all pre-existing in unrelated files).
- [x] `pnpm exec prettier --check` clean.
- [ ] **CI** — `pnpm ci:check` passes on this PR.
- [ ] **Review focus** — the comment block explaining the round-trip rationale (otherwise the `String('literal')` pattern reads like over-engineering).

## Notes for the reviewer

- **Pre-existing warnings are NOT addressed here.** The 13 remaining lint warnings (non-null assertions in `principal-extractor.spec.ts`, unused underscored params in `rate-limit.middleware.ts`, one stale eslint-disable in `downstream-token-cache.service.spec.ts`) are outside the scope of this PR — none of them block CI today, and folding them in would muddle the diff. Worth a separate `chore(bff): sweep lint warnings` PR if/when those become noisy.
- **Why `String()` over an `as string` cast?** Both would work, but `String()` is a real runtime operation (returns a fresh `string`) — `as string` is type-system-only. The runtime call has a tiny side-effect (and the inferred return type really IS `string`, not the literal), so the narrowing test stays semantically real.
- **No new error class.** The lint rule `@typescript-eslint/no-inferrable-types` is set to `error` severity in the workspace config; my narrowing test was just the first case to trip it.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #231

apps/portal-bff/src/structures/structure-kind.spec.ts

@@ -35,7 +35,10 @@ describe('isStructureKind', () => {
   });
 
   it('narrows the type at the call site', () => {
-    const candidate: string = 'medico_social';
+    // Round-trip through `String()` to widen the literal to `string`.
+    // Without this the inferred type is `'medico_social'`, already a
+    // StructureKind subtype — the narrowing check would be vacuous.
+    const candidate = String('medico_social');
     if (!isStructureKind(candidate)) {
       throw new Error('unreachable — candidate is a known kind');
     }