fix(py): fix double generate span in model runner

[huangjeff5]

Dev UI should be calling _generate_action() directly, which runs the generation logic without a new span. Since Dev UI is calling a registered action, there is a generate span already created.

generate_action() is reserved for the ai.generate() case. Since ai.generate() doesn't actually call a registered action, we have to inject the generate span manually there.

[gemini-code-assist]

Code Review

This pull request updates generate_action_fn to call _generate_action directly and refines the documentation for generate_action to clarify span management and prevent redundant nesting in reflection runs. Additionally, it simplifies the comment regarding output format metadata for the Dev UI. I have no feedback to provide as there were no review comments.