fix(bedrock-alpha): added missing validation when prompt uses default variant (#35366)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change
When using Prompts and PromptVariants in bedrock, you can define the default variant your prompt will use, but if you dont add it to the variants array too, it will fail at deployment time.

### Description of changes
Added a validation that checks the correct conditions at synth time, so you don't waste time and leave a stack with a bad deployment

### Describe any new or updated permissions being added
NA


### Description of how you validated changes

Unit tests. We don't create integ tests for validations (synth time) as they wont add any additional value

### 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: alvazjor

packages/@aws-cdk/aws-bedrock-alpha/bedrock/prompts/prompt.ts

@@ -1,4 +1,4 @@
-import { Arn, ArnFormat, IResource, Lazy, Resource } from 'aws-cdk-lib/core';
+import { Arn, ArnFormat, IResource, Lazy, Resource, ValidationError } from 'aws-cdk-lib/core';
 import * as bedrock from 'aws-cdk-lib/aws-bedrock';
 import * as iam from 'aws-cdk-lib/aws-iam';
 import * as kms from 'aws-cdk-lib/aws-kms';
@@ -287,6 +287,7 @@ export class Prompt extends PromptBase implements IPrompt {
     // ------------------------------------------------------
     // Validation
     // ------------------------------------------------------
+    this.validatePromptDefault(props);
     this.node.addValidation({ validate: () => this.validatePromptName() });
     this.node.addValidation({ validate: () => this.validatePromptVariants() });
     this.node.addValidation({ validate: () => this.validateDescription() });
@@ -378,6 +379,16 @@ export class Prompt extends PromptBase implements IPrompt {
     return errors;
   }
 
+  /**
+   * Validates that if the prompt has a default, it was also added to the variants array
+   * @param props - The properties set in the constructor
+   */
+  private validatePromptDefault(props: PromptProps) {
+    if (props.defaultVariant && !props.variants?.includes(props.defaultVariant)) {
+      throw new ValidationError('The \'defaultVariant\' needs to be included in the \'variants\' array.', this);
+    }
+  }
+
   /**
    * Validates whether the description length is within the allowed limit.
    * @returns Array of validation error messages, empty if valid

packages/@aws-cdk/aws-bedrock-alpha/test/bedrock/prompts/prompt.test.ts

@@ -155,32 +155,32 @@ describe('Prompt', () => {
       });
     });
 
-    test('validates prompt name pattern - invalid names', () => {
-      const invalidNames = [
-        '-invalid', // starts with hyphen
-        'invalid-', // ends with hyphen
-        'a'.repeat(101), // too long
-        '', // empty
-        'invalid name', // contains space
-        'invalid@name', // contains special character
-      ];
-
-      invalidNames.forEach((name, index) => {
-        expect(() => {
-          new bedrock.Prompt(stack, `TestPrompt${index}`, {
-            promptName: name,
-          });
-        }).not.toThrow(); // Validation happens at synthesis time, not construction time
+    test.each([
+      ['-invalid', 'starts with hyphen'],
+      ['a'.repeat(101), 'too long'],
+      ['', 'empty'],
+      ['invalid name', 'contains space'],
+      ['invalid@name', 'contains special character'],
+    ])('validates prompt name pattern - %s (%s)', (invalidName, _description) => {
+      new bedrock.Prompt(stack, 'TestPrompt', {
+        promptName: invalidName,
       });
+
+      expect(() => {
+        Template.fromStack(stack);
+      }).toThrow(/Valid characters are a-z, A-Z, 0-9, _ \(underscore\) and - \(hyphen\). Must not begin with a hyphen and must be 1-100 characters long/);
     });
 
     test('validates description length', () => {
+      new bedrock.Prompt(stack, 'TestPrompt', {
+        promptName: 'test-prompt',
+        description: 'a'.repeat(251), // too long
+      });
+
+      // Validation happens at synthesis time, not construction time
       expect(() => {
-        new bedrock.Prompt(stack, 'TestPrompt', {
-          promptName: 'test-prompt',
-          description: 'a'.repeat(251), // too long
-        });
-      }).not.toThrow(); // Validation happens at synthesis time, not construction time
+        Template.fromStack(stack);
+      }).toThrow(/Description must be 200 characters or less/);
     });
 
     test('validates maximum number of variants', () => {
@@ -192,12 +192,15 @@ describe('Prompt', () => {
         }),
       );
 
+      new bedrock.Prompt(stack, 'TestPrompt', {
+        promptName: 'test-prompt',
+        variants,
+      });
+
+      // Validation happens at synthesis time, not construction time
       expect(() => {
-        new bedrock.Prompt(stack, 'TestPrompt', {
-          promptName: 'test-prompt',
-          variants,
-        });
-      }).not.toThrow(); // Validation happens at synthesis time, not construction time
+        Template.fromStack(stack);
+      }).toThrow(/Too many variants specified/);
     });
 
     test('validates unique variant names', () => {
@@ -213,12 +216,30 @@ describe('Prompt', () => {
         promptText: 'Text 2',
       });
 
+      new bedrock.Prompt(stack, 'TestPrompt', {
+        promptName: 'test-prompt',
+        variants: [variant1, variant2],
+      });
+
+      // Validation happens at synthesis time, not construction time
+      expect(() => {
+        Template.fromStack(stack);
+      }).toThrow(/Duplicate variant names found/);
+    });
+
+    test('validates default variant is added to variants array', () => {
+      const defaultVariant = bedrock.PromptVariant.text({
+        variantName: 'default-variant',
+        model: foundationModel,
+        promptText: 'Text',
+      });
+
       expect(() => {
         new bedrock.Prompt(stack, 'TestPrompt', {
-          promptName: 'test-prompt',
-          variants: [variant1, variant2],
+          promptName: 'my-test-prompt',
+          defaultVariant: defaultVariant,
         });
-      }).not.toThrow(); // Validation happens at synthesis time, not construction time
+      }).toThrow('The \'defaultVariant\' needs to be included in the \'variants\' array.');
     });
   });