fix: update Request validation/tests for Employee FK relation

- createRequestSchema now requires employee_id to be a UUID-format
  string (was a free-text non-empty check), matching the new FK to
  Employee in schema.prisma.
- createRequest() now returns a CreateRequestResult discriminated
  union and catches Prisma P2003 (FK violation), returning a friendly
  ok:false error instead of a raw 500 when employee_id does not
  reference an existing Employee. POST /api/requests branches on
  created.ok and returns 400 in that case.
- tests/requests.test.ts: every test that used to POST a fake string
  employee_id (emp-1, emp-2) now creates a real Employee first via
  createTestEmployee() and uses its id, since employee_id is now a
  real FK. Added coverage for a syntactically invalid UUID and for a
  well-formed but non-existent UUID (expects the friendly 400 from
  the P2003 catch, not a raw 500) - the exact case this retrofit was
  meant to protect against.

src/app/api/requests/route.ts

@@ -32,7 +32,11 @@ export async function POST(request: Request): Promise<NextResponse> {
   }
 
   const created = await createRequest(validation.value);
-  return NextResponse.json(created, { status: 201 });
+  if (!created.ok) {
+    return NextResponse.json({ error: created.error }, { status: 400 });
+  }
+
+  return NextResponse.json(created.value, { status: 201 });
 }
 
 export async function GET(): Promise<NextResponse> {