Support `tool_choice` in Agenta prompt templates

[mmabrouk]

Add support for tool_choice parameters in prompts in Agenta.

This includes:

• adding the feature to the SDK for completion and chat
• creating designs or mockups for where the option would be added in the playground for templates
• implementing the frontend changes

Current limitation: the model interface supports tool_choice to control tool behavior, including forcing tool calls, but this is not currently supported in Agenta in the playground or as SDK behavior.

[linear-code]

AGE-3822

[fintech07]

@mmabrouk , can I take this issue?

[mmabrouk]

@fintech07 yes, but please only tackle the SDK part.Validate through tests. Make sure the schema stays aligned with OpenAIs. Make sure also to take into account cases where the model does not support the parameter or the value provided to that field. And please explain in the pr description how different models providers handle this parameter.