fix(models): make tool_choice an optional keyword arg instead positional

[dbschmigelski]

Description

The release of v1.8.0 introduced tool_choice to the Model interface to better support structured_output. This was accidentally added as a positional argument instead of a keyword argument. This is breaking for custom implementations of the Model interface.

The fix here is to switch to an optional keyword arg.

Type of Change

Bug fix
Breaking change

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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