feat(iot-actions-alpha): support batchAcrossTopics in HttpActionBatchConfig

[badmintoncryer]

Issue # (if applicable)

Closes #37980.

Reason for this change

batchAcrossTopics is supported by the underlying L1 AWS::IoT::TopicRule but missing from the L2 HttpActionBatchConfig in @aws-cdk/aws-iot-actions-alpha.

Description of changes

Add optional batchAcrossTopics?: boolean to HttpActionBatchConfig and pass it through to the L1 BatchConfig.BatchAcrossTopics.

Describe any new or updated permissions being added

None.

Description of how you validated changes

• Unit tests covering true / false (via test.each) and the omitted case (Match.absent()).
• Integ test updated with batchAcrossTopics: true and the assertion verifies it in the deployed TopicRule.

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

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.
To suppress a specific rule, see Suppressing Rules.

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	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	48 ran	48 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.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.
To suppress a specific rule, see Suppressing Rules.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	48 ran	48 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates