fix(elasticloadbalancingv2): improve handling for dropInvalidHeaderFields in case of default or switching from true to false

[rgoltz]

Issue # (if applicable)

Closes #36409

Reason for this change

Previously, the dropInvalidHeaderFields property (in CFN routing.http.drop_invalid_header_fields.enabled) was only set in the CloudFormation (CFN) template when explicitly set to true. When set to false, the attribute was omitted from the template, making it impossible for CloudFormation to detect configuration drift or properly update the ALB when the property value changed from true to false.

Description of changes

• Modified dropInvalidHeaderFields behavior: Changed from conditional setting (only when true) to explicit setting for both true and false values. This should help CloudFormation change set and/or drift detection as well (I love CFN drift detection 🫶).

• Added tests: Created test cases covering both true/false values and undefined behavior to ensure proper CloudFormation template generation

Description of how you validated changes

Tried to ensured existing functionality remains unchanged while adding support for explicit false values.

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

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

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	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	25 ran	25 passed

Test	Result
No test annotations available

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	25 ran	25 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates

[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.

[rgoltz]

Clarification Request