chore: security-guardian cfn-guard rule with updated rule scenarios (#34334)

### Issue # (if applicable)
Fixes issues when AWS::IAM::Role doesnt exist in the snapshot templates in the PR and yet security guardian complained incorrectly.

Closes #<issue number here>.

### Reason for this change



### Description of changes



### Describe any new or updated permissions being added




### Description of how you validated changes



### Checklist
- [ x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: QuantumNeuralCoder

tools/@aws-cdk/security-guardian/rules/trust_scope_rules.guard

@@ -1,28 +1,51 @@
-# Trust Scope Security Rules
-# This rule file checks for overly broad trust scopes in IAM resources
+#
+#####################################
+##           AWS CDK               ##
+#####################################
+# Rule Identifier:
+#    IAM_ROLE_NO_BROAD_PRINCIPALS
+#
+# Description:
+#   Checks if IAM roles have overly permissive assume role policies by identifying:
+#   1. Use of account root in AWS principals
+#   2. Use of wildcards in AWS principals
+#   3. Use of wildcards as entire principal
+#   4. Allows specific role ARNs that are not root
+#   5. Catches use of !Join function to construct root ARNs
+#
+# Reports on:
+#    AWS::IAM::Role
+#
+# Evaluates:
+#    AWS CloudFormation
+#
+# Rule Parameters:
+#    None
+#
+# Scenarios:
+# a) SKIP: when there are no IAM Role resources
+# b) SKIP: when IAM Role resources only have service principals
+# c) PASS: when IAM Role resources with AWS principals use specific ARNs (not root or wildcards)
+# d) PASS: when IAM Role resources with AWS principals use specific role ARNs
+# e) FAIL: when any IAM Role resource uses account root in AWS principal
+# f) FAIL: when any IAM Role resource uses wildcard in AWS principal
+# g) FAIL: when any IAM Role resource uses wildcard as entire principal
 
-# Rule to check for overly permissive IAM role trust policies
-rule iam_role_trust_policy_not_overly_permissive {
-  when AWS::IAM::Role exists {
-    Properties exists
-    Properties is_struct
-    
-    Properties.AssumeRolePolicyDocument exists
-    Properties.AssumeRolePolicyDocument is_struct
-    
-    Properties.AssumeRolePolicyDocument {
-      Statement exists
-      Statement is_list
-      
-      # For each statement in the policy
-      Statement[*] {
-        # Check if Principal is overly permissive
+#
+# Select all IAM Role resources from incoming template
+#
+let iam_roles_no_broad_principals = Resources.*[ Type == 'AWS::IAM::Role'
+  Metadata.guard.SuppressedRules not exists or
+  Metadata.guard.SuppressedRules.* != "IAM_ROLE_NO_BROAD_PRINCIPALS"
+]
+
+rule IAM_ROLE_NO_BROAD_PRINCIPALS when %iam_roles_no_broad_principals !empty {
+    %iam_roles_no_broad_principals.Properties.AssumeRolePolicyDocument.Statement[*] {
         when Principal exists {
-          # Check if Principal is a string (direct "*" case)
-          when Principal is_string {
-            Principal != "*"
-          }
-          
+            # Check for wildcard as entire principal
+            when Principal is_string {
+              Principal != "*"
+            }
           # Check if AWS principal exists
           when Principal.AWS exists {
             # Check if AWS is a string
@@ -31,9 +54,6 @@ rule iam_role_trust_policy_not_overly_permissive {
               Principal.AWS != /(?i):root/
             }
           }
-        }
-      }
     }
   }
-}
-
+}