#46752: Fix missing offset assignment logic in checkpoint store

[kamil-di]

Description

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

If an SDK is being regenerated based on a new swagger spec, a link to the pull request containing these swagger spec changes has been included above.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Thank you for your contribution @kamil-di! We will review the pull request and get back to you soon.

[Copilot]

Pull Request Overview

This PR fixes a bug in the blob checkpoint store where offset assignment logic was missing or incomplete, ensuring that checkpoint offset values are properly handled when updating checkpoints in blob storage.

• Fixed missing offset assignment logic to handle both string and numeric offset values
• Added proper null/empty string validation for offset values
• Ensured metadata is correctly populated with offset information

[Copilot]

There are duplicate assignments to the metadata map. Lines 268 and 270 both put SEQUENCE_NUMBER, and lines 269 and 271 both put OFFSET. The second assignments (lines 270-271) will overwrite the first ones, making the new offset logic ineffective. Lines 270-271 should be removed.

Suggested change

	metadata.put(SEQUENCE_NUMBER, sequenceNumber);
	metadata.put(OFFSET, checkpoint.getOffsetString());