[ServiceBus] Add maxMessageSizeInKilobytes to swagger

[yunhaoling]

resolving issue: Azure/azure-sdk-for-python#20668

[openapi-workflow-bot]

Hi, @yunhaoling Thanks for your PR. I am workflow bot for review process. Here are some small tips.

Please ensure to do self-check against checklists in first PR comment.

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[openapi-pipeline-app]

Swagger Validation Report

️⌛BreakingChange pending [Detail]

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️⌛ModelValidation pending [Detail]

️⌛SemanticValidation pending [Detail]

️⌛Cross-Version Breaking Changes pending [Detail]

️⌛[Staging] SDK Track2 Validation pending [Detail]

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[openapi-pipeline-app]

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_{Posted by Swagger Pipeline | How to fix these errors?}