server : fix 500 bad_function_call on validation errors with X-Conversation-Id (#25605)

[AmariahAK]

When POST /v1/chat/completions carries the X-Conversation-Id header and the request body fails validation (e.g. "backend_sampling": ""), the server returns 500 bad_function_call instead of 400. The same invalid body without the header correctly returns 400.
Root cause: server_res_spipe::set_req() attaches the stream pipe before validation. When validation throws, set_next() is never called, leaving next_orig empty. The error path still calls on_complete(), which invokes the empty std::function and throws std::bad_function_call. That escapes to httplib's exception handler, which replaces the 400 JSON body with 500 plain-text.
Fix: add !next_orig to the guard in server_res_spipe::on_complete() so it returns early when next_orig was never assigned.
AI usage disclosure: YES — AI was used to analyze the bug and suggest the fix. The change was reviewed and understood before committing.

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