fix(aspects): "localAspects is not iterable" error (#32647)

Closes #32470 

### Reason for this change

Some customers have reported seeing the error `TypeError: localAspects is not iterable` upon upgrading to CDK v2.172.0 (this is when the Priority-ordered aspects feature was released). This is likely caused by customers having dependencies on third-party constructs/libraries which are using outdated versions (< 2.172.0) of CDK. The problem more specifically is that the `Aspects.applied` function was added in v2.172.0, and the new `invokeAspects` function calls this function on all nodes in the tree.

### Description of changes

Created a workaround for customers. Added the `getAspectApplications` function in `synthesis.ts` - this function creates `AspectApplication` objects from `Aspects.all` if `Aspects.applied` does not exist.

### Describe any new or updated permissions being added

None.

### Description of how you validated changes

New unit test in `aspect.test.ts` with a monkey patched `Aspects.applied` function.

### 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*

packages/aws-cdk-lib/core/lib/private/synthesis.ts

@@ -8,7 +8,7 @@ import { CloudAssembly } from '../../../cx-api';
 import * as cxapi from '../../../cx-api';
 import { Annotations } from '../annotations';
 import { App } from '../app';
-import { AspectApplication, Aspects } from '../aspect';
+import { AspectApplication, AspectPriority, Aspects } from '../aspect';
 import { FileSystem } from '../fs';
 import { Stack } from '../stack';
 import { ISynthesisSession } from '../stack-synthesizers/types';
@@ -228,7 +228,7 @@ function invokeAspects(root: IConstruct) {
     const node = construct.node;
     const aspects = Aspects.of(construct);
 
-    let localAspects = aspects.applied;
+    let localAspects = getAspectApplications(construct);
     const allAspectsHere = sortAspectsByPriority(inheritedAspects, localAspects);
 
     const nodeAspectsCount = aspects.all.length;
@@ -290,11 +290,10 @@ function invokeAspectsV2(root: IConstruct) {
 
   function recurse(construct: IConstruct, inheritedAspects: AspectApplication[]): boolean {
     const node = construct.node;
-    const aspects = Aspects.of(construct);
 
     let didSomething = false;
 
-    let localAspects = aspects.applied;
+    let localAspects = getAspectApplications(construct);
     const allAspectsHere = sortAspectsByPriority(inheritedAspects, localAspects);
 
     for (const aspectApplication of allAspectsHere) {
@@ -354,6 +353,21 @@ function sortAspectsByPriority(inheritedAspects: AspectApplication[], localAspec
   return allAspects;
 }
 
+/**
+ * Helper function to get aspect applications.
+ * If `Aspects.applied` is available, it is used; otherwise, create AspectApplications from `Aspects.all`.
+ */
+function getAspectApplications(node: IConstruct): AspectApplication[] {
+  const aspects = Aspects.of(node);
+  if (aspects.applied !== undefined) {
+    return aspects.applied;
+  }
+
+  // Fallback: Create AspectApplications from `aspects.all`
+  const typedAspects = aspects as Aspects;
+  return typedAspects.all.map(aspect => new AspectApplication(node, aspect, AspectPriority.DEFAULT));
+}
+
 /**
  * Find all stacks and add Metadata Resources to all of them
  *

packages/aws-cdk-lib/core/test/aspect.test.ts

@@ -309,4 +309,24 @@ describe('aspect', () => {
       }
     }
   }
+
+  test.each([
+    { stabilization: true },
+    { stabilization: false },
+  ])('Error is not thrown if Aspects.applied does not exist (stabilization: $stabilization)', ({ stabilization }) => {
+    const app = new App({ context: { '@aws-cdk/core:aspectStabilization': stabilization } });
+    const root = new Stack(app, 'My-Stack');
+
+    Aspects.of(root).add(new Tag('AspectA', 'Visited'));
+
+    // "Monkey patching" - Override `applied` to simulate its absence
+    Object.defineProperty(Aspects.prototype, 'applied', {
+      value: undefined,
+      configurable: true,
+    });
+
+    expect(() => {
+      app.synth();
+    }).not.toThrow();
+  });
 });