Proposal: Pass error context to ResponseComplete extension point

[LukeAVanDrie]

What would you like to be added:

I propose updating the ResponseComplete plugin interface signature to include an error parameter (or a structured status object).

Currently, the signature is:

ResponseComplete(ctx context.Context, request *types.LLMRequest, response *Response, targetPod *backend.Pod)

It should be updated to something resembling:

ResponseComplete(..., err error)
// OR
ResponseComplete(..., status TerminationStatus)
// OR
ResponseComplete(..., reason string)

Why is this needed:

With the recent merge of PR #2064, the ResponseComplete hook is now guaranteed to execute symmetrically with PreRequest, covering all termination states:

1. Successful completion.
2. Internal errors (e.g., JSON marshaling failures).
3. Client disconnects / Context cancellation.

The Problem:
Currently, a plugin implementing this hook has no way to distinguish between a successful request and a cancelled/failed one. This limitation prevents plugins from accurately recording observability data (e.g., incrementing an error_count metric vs. a success_count, or logging the cause of a disconnect).

Notes:

• This is a breaking change for existing plugins.