chore(cognito): validate custom attribute names don't conflict with standard attributes

[pahud]

Issue # (if applicable)

Closes #31593.

Reason for this change

Custom attributes in Cognito User Pools with names matching standard attributes (e.g., name, email, phone_number) were silently dropped by CloudFormation during deployment. This led to runtime failures with no clear indication of the problem, making it difficult for developers to diagnose the issue.

Description of changes

Added synthesis-time validation in the UserPool.schemaConfiguration method to detect name collisions between custom attributes and standard attributes:

• Validates custom attribute names against all 20 standard attribute names during CDK synthesis
• Throws a clear ValidationError with the conflicting attribute name and a complete list of standard attributes
• Properly handles CloudFormation Tokens (parameters) by skipping validation for unresolved values
• Provides actionable error messages that guide developers to rename conflicting attributes

Example error message:

Custom attribute 'name' conflicts with a standard attribute name. 
Custom attributes cannot use the same name as standard attributes. 
Standard attribute names: address, birthdate, email, family_name, gender, 
given_name, locale, middle_name, name, nickname, phone_number, picture, 
preferred_username, profile, zoneinfo, updated_at, website, email_verified, 
phone_number_verified

Standard attributes validated (20 total):
address, birthdate, email, family_name, gender, given_name, locale, middle_name, name, nickname, phone_number, picture, preferred_username, profile, zoneinfo, updated_at, website, email_verified, phone_number_verified

Breaking changes: None. This change only affects code that was already broken (silent failures). Existing valid configurations continue to work unchanged.

Describe any new or updated permissions being added

N/A - No IAM permissions or resource access changes.

Description of how you validated changes

• Unit tests: Added 5 comprehensive test cases covering:

  • Single collision detection (e.g., custom attribute named name)
  • Multiple collision detection across different standard attribute names
  • Valid non-conflicting custom attribute names
  • CloudFormation parameter handling (Token.isUnresolved)
  • Error message completeness and clarity

• Test results: All 344 existing tests pass (100% success rate), plus 5 new tests

• Regression testing: Verified no impact on existing functionality

• JSII compatibility: Confirmed ValidationError works across all language bindings

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