fix(core): skip default validation when context is unresolved - #39560

Issue # (if applicable)

Closes #38989.

Reason for this change

The built-in CloudFormation validation plugin reports false-positive violations during synthesis when context lookups have not yet resolved. CDK generates temporary placeholder values such as vpc-12345, p-12345, and dummy-value-for-..., which fail CloudFormation's resource identifier format checks.

These warnings can appear during unit tests and initial synthesis, even though the placeholder values are not intended for deployment.

Description of changes

• Detect unresolved context across synthesized cloud assemblies.
• Skip automatic registration of the built-in CloudFormation validation plugin when context is missing.
• Preserve explicitly registered validation plugins.
• Retain normal built-in validation when context is fully resolved.
• Add regression coverage verifying that missing context suppresses automatic validation while a custom plugin continues running.

This addresses the false positives centrally without modifying the existing lookup placeholders.

Describe any new or updated permissions being added

None.

Description of how you validated changes

• Reproduced the original CloudFormation validation warnings using the issue's VPC and AMI lookup scenario.
• Added a regression test for unresolved context and custom plugin execution.
• Passed all 43 tests in cloudformation-validate-plugin.test.ts.
• Successfully built aws-cdk-lib and its dependencies: 10 build projects passed.
• Verified formatting and lint checks.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES.

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