chore(core): introduce DetachedConstruct base class

[mrgrain]

Reason for this change

Refactor repeated pattern of anonymous classes that throw UnscopedValidationError when accessing node or env properties.

This pattern exists because certain legacy CDK APIs like ManagedPolicy.fromAwsManagedPolicyName() and CloudFront's fromManagedCachePolicy() return construct-like objects without requiring a scope parameter. These APIs predate modern CDK patterns where all constructs require a scope. They cannot be changed without breaking existing customer code, so they return "detached" constructs that throw errors when used in APIs that require a proper construct tree context.

But we also sometimes add new features to resource interfaces like IManagedPolicy. When this happens, we have to update this fake implementation in a number of places. It also prevents us (and users) from providing augmentations for the IConstruct interface like proposed in #36013.

With this change, we have reduced the number of places to update to one. It's now also possible to augment IConstruct.

Description of changes

• Created DetachedConstruct base class in core/lib/private/detached-construct.ts that encapsulates the pattern of throwing UnscopedValidationError when accessing node or env
• Refactored CloudFront policy classes (CachePolicy, OriginRequestPolicy, ResponseHeadersPolicy) to extend DetachedConstruct instead of implementing the pattern inline
• Refactored ManagedPolicy.fromAwsManagedPolicyName() to use DetachedConstruct
• Refactored ElasticBeanstalkDeployAction to use DetachedConstruct for managed policy reference
• Added unit tests for DetachedConstruct to verify error throwing behavior

This change reduces code duplication and makes the pattern more maintainable. The DetachedConstruct class is marked as internal and documented to discourage use in new APIs.

Describe any new or updated permissions being added

No new or updated permissions.

Description of how you validated changes

• Added unit tests in core/test/private/detached-construct.test.ts that verify UnscopedValidationError is thrown when accessing node and env properties
• Existing tests continue to pass, ensuring no behavioral changes

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

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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

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