fix: move unreachable error event check before thread event guard in streaming

[giulio-leone]

Summary

Fixes #2796

The sse.event == "error" check inside both Stream.__stream__() and AsyncStream.__stream__() was nested inside the if sse.event.startswith("thread.") block. Since "error" never starts with "thread.", the error-event check was unreachable dead code.

Changes

Move the error-event check to execute before the thread-event guard so that SSE error events are properly detected regardless of event name prefix. The data-level error check in the else branch is preserved as a second line of defence for non-thread events.

Before

if sse.event and sse.event.startswith("thread."):
    data = sse.json()
    if sse.event == "error" ...  # ← unreachable: "error" never starts with "thread."
        raise APIError(...)
    yield ...
else:
    ...

After

if sse.event == "error":
    data = sse.json()
    if is_mapping(data) and data.get("error"):
        raise APIError(...)

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

Applied to both Stream.__stream__() (sync) and AsyncStream.__stream__() (async).

Testing

All 20 streaming tests pass (2 consecutive clean runs).

[giulio-leone]

Closing — this unreachable code check is too marginal to warrant a PR.