fix mint fallback admin tokens

-    string username = "";
+    string username;
     if payload.hasKey(IDP_CLAIMS) {
         json idpClaims = <json>payload.get(IDP_CLAIMS);
-        json|error un = idpClaims.username;
-        username = un is json ? un.toString() : "";
+        json|error un = idpClaims.username;
+        if un is error {
+            return error("Username claim is missing");
+        }
+        username = un.toString();
     } else if payload.sub is string {
-        username = payload.sub ?: "";
+        username = payload.sub ?: "";
+        if username == "" {
+            return error("Subject claim is missing");
+        }
+    } else {
+        return error("Required identity claims are missing");
     }

-    json crTokenPayload = {"sub": username, "role": "admin", "exp": 9999999999999};
+    // derive role from trusted claim/context instead of hardcoding
+    json crTokenPayload = {"sub": username, "role": "viewer", "exp": 9999999999999};

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@iol/iol-core/utils.bal` around lines 50 - 59, The code currently constructs
crTokenPayload with a potentially empty username and a hardcoded "admin" role;
change this to fail closed when no valid username is present and stop minting a
fallback token. Specifically, in the block that reads payload, ensure username
(from IDP_CLAIMS or payload.sub) is validated and if empty or not a trusted
claim, return an error/abort instead of proceeding; remove the hardcoded
role="admin" in crTokenPayload and populate the role only from verified/trusted
claims or context (e.g., derived from idpClaims or a trusted auth context)
before creating the payload.