feat: prompt pre fetch plugin call

[julianstephen]

Summary

Fixes for #56

Related issue(s)

(Optional) Testing Instructions

Create a new envoy filter that will route to ext proc on prompt gets

[rubambiza]

Thanks for adding prompt pre-fetch support — the overall structure looks good. A few things to address before this can merge:

CI is red — build fails on both Python 3.11 and 3.12. Please investigate and fix.

assert payload.args is not None (plugin.py ~line 90) — assert statements are stripped in optimized mode (python -O) and will crash the process otherwise. This should be a proper check that returns a PromptPrehookResult with a violation, similar to the error handling below it.

Unreachable code — the original return PromptPrehookResult(continue_processing=True) at the end of prompt_pre_fetch is now dead code after the new implementation. Can be removed.

Minor:

• f-string f"... payload {payload}" in the debug log could dump large objects. Consider logging just payload.prompt_id or similar.
• Missing newline at end of tests/single-server-filter.yaml.

[rubambiza]

@julianstephen Since this is still in draft, feel free to ignore my comments for now 😅 I should have specified to Claude to surface PRs that are ready for reviews.