fix: handle param.Opt types in ChatCompletionToolChoiceOptionUnionParam unmarshaling

[akar016012]

This commit addresses issue #520 where the JSON unmarshaling fails for ChatCompletionToolChoiceOptionUnionParam when encountering OfAuto field with param.Opt[string] type. The decoder.field.Type.Elem() method was calling .Elem() on param.Opt[string] which is not a pointer type, causing unmarshaling to fail.

Modified files and changes:

• chatcompletion.go:
  • Added custom UnmarshalJSON method for ChatCompletionToolChoiceOptionUnionParam
  • Implemented proper handling of simple string values for OfAuto field
  • Added discriminated object unmarshaling based on "type" field
  • Added support for "function", "allowed_tools", and "custom" types
  • Replaced generic apijson.UnmarshalRoot call with type-specific handling

This fix ensures that the union parameter can properly unmarshal both string values and complex objects without encountering type reflection errors when dealing with param.Opt types.

[akar016012]

Alternate solution PR : https://github.com/openai/openai-go/pull/528