fix(region-info): facts only returned from constant region list (#27506)

Facts are only being returned from the regions in a constant list.
This PR returns facts for all Regions in `AWS_REGIONS` + `this.database`.

Closes #27260.

----

*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/region-info/lib/fact.ts

@@ -5,12 +5,12 @@ import { AWS_REGIONS } from './aws-entities';
  */
 export class Fact {
   /**
-   * @returns the list of names of AWS regions for which there is at least one registered fact. This
-   *          may not be an exhaustive list of all available AWS regions.
+   * @returns the list of names of AWS Regions for which there is at least one registered fact. This
+   *          includes Regions defined in AWS_REGIONS plus custom defined regions.
    */
   public static get regions(): string[] {
-    // Return by copy to ensure no modifications can be made to the undelying constant.
-    return Array.from(AWS_REGIONS);
+    // Return the union of regions in AWS_REGIONS and custom defined regions.
+    return [...new Set([...AWS_REGIONS, ...Object.keys(this.database)])];
   }
 
   /**
@@ -37,7 +37,7 @@ export class Fact {
     const foundFact = this.find(region, name);
 
     if (!foundFact) {
-      throw new Error(`No fact ${name} could be found for region: ${region} and name: ${name}`);
+      throw new Error(`No fact ${name} could be found for region: ${region} and name: ${name}.`);
     }
 
     return foundFact;

packages/aws-cdk-lib/region-info/test/fact.test.ts

@@ -89,4 +89,23 @@ describe('register', () => {
     // Cleanup
     Fact.unregister(region, name);
   });
+
+  test('registering a fact with a new region adds the region', () => {
+    // GIVEN
+    const region = 'my-custom-region';
+    const name = FactName.PARTITION;
+    const value = 'nebraska';
+
+    // WHEN
+    expect(Fact.find(region, name)).not.toBe(value);
+    expect(() => Fact.register({ region, name, value })).not.toThrowError();
+
+    // THEN
+    expect(Fact.regions.includes('my-custom-region')).toBeTruthy();
+    expect(Fact.find(region, name)).toBe('nebraska');
+  });
+
+  test('regions does not return duplicate regions', () => {
+    expect(new Set(Fact.regions).size == Fact.regions.length).toBeTruthy();
+  });
 });

packages/aws-cdk-lib/region-info/test/region-info.test.ts

@@ -72,4 +72,4 @@ test.each([
   ['us-iso-west-1', false],
 ])('%p should be opt-in: %p', (region, expected) => {
   expect(RegionInfo.get(region).isOptInRegion).toEqual(expected);
-});
+});