fix(elbv2): clarify imported target group listener warning

[caglarkc]

Issue # (if applicable)

Closes #31103.

Description

Attaching a listener to an imported application target group works, but the warning incorrectly says the listener cannot be registered. Explain the actual limitation: CDK cannot automatically update the targets' security groups. Keep the existing warning identifier so acknowledgements continue to work.

Validation

• Application target group unit tests: 81 passed, including the regression that asserts both the listener forward action and the security-group warning.
• ESLint passed for both changed TypeScript files.

Integration-test exemption requested: this changes only diagnostic wording, without changing deployment templates or runtime behavior.

---

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:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

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.

[maintainer-for-aws]

Automated review

A maintainer will still review this — treat the notes below as a starting point.

📌 Commit f13f981

This PR clarifies a misleading synthesis-time warning on ImportedApplicationTargetGroup.registerListener in the elbv2 package. It rewords a single Annotations.of(this).addWarningV2(...) message so it accurately describes the real limitation (CDK cannot auto-manage target security groups for an imported target group) instead of the inaccurate "cannot register listener," and adds a regression test. The stable acknowledgement id (@aws-cdk/aws-elbv2:albTargetGroupCannotRegisterListener) is preserved, so users who already suppressed this warning keep it suppressed, and no CloudFormation output or runtime behavior changes. The new test locks in both the positive listener forward-action synthesis and the exact warning text with the preserved ack id, guarding against a future accidental removal. No issues found on this change.

🔴 0 blocking · 🟡 0 recommended · ⚪ 0 optional

---

Generated automatically. React 👍 or 👎 to tell us whether this review helped, so we can improve these reviews.

[caglarkc]

Could a maintainer consider pr-linter/exempt-integ-test for this warning-text correction? It preserves the warning ID and deployment template. The 81 target-group tests pass, including the regression that verifies listener forwarding still works and the warning accurately describes manual security-group updates.