chore(py): flatten prompt opts to kwargs

[huangjeff5]

Previously, callers had to bundle generation options into a single opts dict:

response = await prompt(input, opts={"model": "...", "config": {...}})

This PR flattens those options into keyword arguments using Unpack[PromptGenerateOptions]:

response = await prompt(input, model="...", config={...})

This way we still have a shared type for the stream/non-streaming path, but the syntax is more pythonic (using kwargs instead of an opts dict), and all the various prompt options should surface correctly inline in the IDE (as opposed to being hidden behind an opaque type where you have to further inspect to see the real fields).

[gemini-code-assist]

Code Review

This pull request refactors the ExecutablePrompt API to transition from a single opts dictionary to using keyword arguments for generation options, leveraging Unpack[PromptGenerateOptions] for type safety. The implementation introduces internal helper methods _call_impl and _render_impl to consolidate logic, along with a _coerce_prompt_opts utility that validates input and provides explicit error messages if the deprecated opts parameter is used. Corresponding test cases have been updated and expanded to verify the new keyword-based interface and the removal of the legacy parameter. I have no feedback to provide.