feat(openai): support object-type FunctionCallBehavior for forcing a …

[ljluestc]

…specific function

The OpenAI API allows function_call to be either a string ("none", "auto") or an object ({"name": "my_function"}) to force the model to call a particular function. Previously, FunctionCallBehavior was typed as a string and could not represent the object form.

Changes:

• Change ChatRequest.FunctionCallBehavior from FunctionCallBehavior (string) to any in openaiclient, enabling both string and object forms
• Change CallOptions.FunctionCallBehavior from FunctionCallBehavior to any in the llms package for the same reason
• Add FunctionCallBehaviorFunctionObject struct with Name field
• Add WithFunctionCallBehaviorFunction(name string) CallOption that sets function_call to {"name": ""} in the OpenAI request
• Remove now-unnecessary type cast in openaillm.go
• Add unit tests for the new option and JSON serialization of all forms

Existing string-valued behavior ("none", "auto", unset) is unchanged. The deprecated function_call field is superseded by tool_choice in the modern OpenAI API; users should prefer WithToolChoice for new code.

Closes #254

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