test: fix flaky openai structured output test by adding Field guidance

[dbschmigelski]

Description

The following test failed in https://github.com/strands-agents/sdk-python/actions/runs/21082854772.

Anecdotally, I have seen this test fail before. I believe the additional description will reduce the flakiness of this test. I ran 10 times and it passed all 10 locally.

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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