fix: remove unreachable error check inside thread event block

[ZAID646]

In src/openai/_streaming.py, both the sync and async __stream__ methods contain a dead code block:

if sse.event and sse.event.startswith("thread."):
    data = sse.json()

    # DEAD CODE: "error" does not start with "thread.", so this check is unreachable
    if sse.event == "error" and is_mapping(data) and data.get("error"):
        ...
        raise APIError(...)

    yield process_data(...)

The sse.event == "error" check inside the sse.event.startswith("thread.") block can never be true. This was a regression from commit abc25966 where the error check was incorrectly moved inside the thread branch during a refactor. The else branch already handles errors correctly for non-thread events.

Fixes #2796