[WIP] Add tool_choice to ChatCompletionClient create and create_stream method

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Issue body

The tool_choice parameter should accept the following:

• a list of strings corresponding to the names of the tools that will be provided to the model as the choices
• a list of Tool types, which also will be converted to a list of strings as their names to be used as the choices

For each model client implementation, use the model API’s convention for the actual create argument passed to the model API. For example, the argument name is tool_choice for OpenAI API.

Related #6695

Fixes #6696.

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