fix: resolve union decoder field type reflection for param.Opt types #528
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
This commit fixes issue #520 by improving the union decoder's ability to handle param.Opt types
during JSON unmarshaling. The problem occurred when the decoder tried to call .Elem() on
param.Opt[string] types, which are not pointer types.
Modified files and changes:
chatcompletion.go:
internal/apijson/union.go:
This fix ensures that union parameters can be properly decoded without type reflection errors,
specifically addressing cases where union fields use param.Opt types instead of pointer types.