Added missing openai_client reference to sample

[furoTmark]

I think this was a missing piece as the sample used openai_client, but it wasn't defined anywhere.
Maybe a link to the samples could also help.

[furoTmark]

@microsoft-github-policy-service agree

[prmerger-automator]

@furoTmark : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit d072f67:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/ai-foundry/default/agents/how-to/migrate.md	✅Succeeded

For more details, please refer to the build report.

[ktoliver]

@aahill

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

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Pull request overview

This PR fixes a code sample in the migration guide by adding the missing initialization of the openai_client variable. The sample code was using openai_client without defining it, which would cause an error if users tried to run the code as-is.

Key changes:

• Wrapped the code sample in a with project_client.get_openai_client() as openai_client: context manager to properly initialize the openai_client
• Re-indented all code within the sample to reflect the new context manager scope

[learn-build-service-prod]

Learn Build status updates of commit e9667d2:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/ai-foundry/default/agents/how-to/migrate.md	✅Succeeded

For more details, please refer to the build report.