Changelog update for missing duration catch

[kingernupur]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

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All SDK Contribution checklist:

• [ x] The pull request does not introduce [breaking changes]
• [x ] CHANGELOG is updated for new features, bug fixes or other significant changes.
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• [x ] Title of the pull request is clear and informative.
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Testing Guidelines

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[Copilot]

Pull Request Overview

This pull request updates the CHANGELOG to document a bug fix addressing a missing duration value in the deployment poller result.

• Added a new changelog entry for version 1.26.1 dated 2025-03-27
• Describes the bug fix for handling missing duration value

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-ai-ml