fix: Handle empty choices in OpenAI model provider

[mrityunjayshukla]

Description

Fixes bug 136 IndexError: list index out of range in src/strands/models/openai.py during streaming, we should guard against empty choices lists in the OpenAI response. This pattern is seen in other repos and is a common defensive fix for OpenAI's streaming API, which may emit events without any choices.

Related Issues

136

Documentation PR

N/A

Type of Change

• Bug fix

Testing

[How have you tested the change?]

• hatch fmt --linter
• hatch fmt --formatter
• hatch test --all
• Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

Checklist

• I have read the CONTRIBUTING document
• I have added 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
• My changes generate no new warnings
• Any dependent changes have been merged and published

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