docs(s3,kms): replace object-literal IResourceWithPolicyV2 example with named class

[AmariahAK]

Description

Closes #38452

Reason for this change

The IResourcePolicyFactory / IResourceWithPolicyV2 usage example in the S3
and KMS READMEs used a TypeScript object-literal-with-inline-method pattern
for the return value of forResource():

return {
  env: resource.env,
  addToResourcePolicy(statement) { ... }
};

While this satisfies the structural interface in TypeScript, jsii/Rosetta
cannot translate anonymous objects with methods to other languages. The
generated Ruby docs showed an inline def inside a hash, Python had raw
function bodies inside dicts, and Java produced Map.of() with a broken
method body. See the full breakdown in #38452.

Description of changes

Updated the nofixture code blocks in both READMEs to use a proper named
class MyResourceWithPolicy that explicitly implements IResourceWithPolicyV2:

• packages/aws-cdk-lib/aws-s3/README.md — replaced object-literal with
  MyResourceWithPolicy class
• packages/aws-cdk-lib/aws-kms/README.md — identical fix, adapted for
  AWS::KMS::Key

Both now follow the same class-based pattern as CfnBucketWithPolicy and
CfnKeyWithPolicy in their respective default-traits.ts source files.

Additional imports added: AddToResourcePolicyResult (from
aws-cdk-lib/aws-iam) and ResourceEnvironment (from
aws-cdk-lib/interfaces). Removed unused IConstruct import.

Note: this is NOT the full fix for #38452. This PR only addresses the
README examples. The Rosetta transpiler itself still has bugs in the C#,
Golang, and Java targets (e.g. resource = resource; self-assignment in C#,
incorrect pointer usage in Go, missing getEnv() in Java) that will need a
separate PR in the Rosetta codebase. See @omarqureshi's analysis in the issue
for details on those remaining transpiler issues.

Describe any new or updated permissions being added

None — documentation-only change.

Description of how you validated changes

• Verified the new class structure matches the existing CfnBucketWithPolicy
  and CfnKeyWithPolicy implementations in default-traits.ts exactly
• Confirmed all imported types (AddToResourcePolicyResult,
  ResourceEnvironment, IResourceWithPolicyV2, IResourcePolicyFactory,
  PolicyStatement, ResourceWithPolicies) are exported from their
  respective modules
• Confirmed the issue author's Rosetta translation output shows clean,
  idiomatic Ruby, Python, and Java for the class-based pattern
• Full Rosetta extraction (yarn rosetta:extract --strict) should be run
  on a machine with ≥16GB RAM to verify cross-language translation before
  merge

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

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ The title prefix of this pull request must be one of "feat|fix|build|chore|ci|docs|style|refactor|perf|test|revert"

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

[omarqureshi]

@AmariahAK - for cleanliness, instead of a merge from main, could you rebase instead and push (force with lease) a single squashed commit?

[AmariahAK]

@omarqureshi should be good now