fix(elasticache-alpha): deployment fails when serverlessCacheName or userGroupId is not specified

[mazyu36]

Issue # (if applicable)

Closes #36458

Reason for this change

When deploying an ElastiCache ServerlessCache or UserGroup without explicitly specifying a serverlessCacheName or userGroupId, the auto-generated name can exceed AWS ElastiCache's 40-character limit, causing deployment failures.

The current implementation uses Names.uniqueId() which generates names without length constraints. With longer stack names or construct paths, the generated identifier exceeds the service's maximum allowed length.

Description of changes

Replace the name generation logic with Names.uniqueResourceName(this, { maxLength: 40 }) to ensure generated names respect the AWS limit:

Description of how you validated changes

Added integration test integ.serverless-cache-auto-generate-name.ts that creates a ServerlessCache and UserGroup without specifying names.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

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	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	50 ran	50 passed

Test	Result
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[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	50 ran	50 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates

[mazyu36]

Exemption Request: Existing unit tests already cover auto-generated names, and the integration test validates the 40-character limit in the generated snapshot.