fix a bug of openai provider stop reason

[yytdfc]

fix a bug of openai provider stop reason

Description

fix a bug of openai provider that doesn't privide the right stop reason for tool call. (such as SGLang)

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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