fix(api): widen asString return type to string

asString returned the ZodEnum unchanged, so z.infer leaked the literal
union. Consumers bind their own zod for the @anatine/zod-openapi and
@ts-rest/core peers; under zod 4 the union breaks assignments from plain
string data (e.g. certification, gender, known_for_department). Annotate
the return as z.ZodString so the inferred output stays string regardless
of the consumer's zod. Runtime still validates enum membership and the
OpenAPI schema stays type: string.

Author: vladjerca

projects/api/src/contracts/_internal/z.ts

@@ -33,9 +33,14 @@ export function int64(schema: z.ZodNumber) {
  */
 export function asString<T extends z.ZodEnum<[string, ...string[]]>>(
   schema: T,
-) {
-  // Use type assertion to access the openapi method
-  return (schema as T & { openapi: (meta: unknown) => T })
+): z.ZodString {
+  // The inferred output must stay `string`, not the literal union: consumers
+  // bind their own zod for the @anatine/zod-openapi peer, and under zod 4 the
+  // enum would otherwise leak as a literal union and break assignments from
+  // plain string data. Runtime keeps validating enum membership.
+  return (schema as unknown as z.ZodString & {
+    openapi: (meta: unknown) => z.ZodString;
+  })
     .openapi({
       type: 'string',
       enum: undefined,