fix(streaming): move error event check before thread.* handling

[yurekami]

Summary

Fixes #2796

The sse.event == "error" check inside the sse.event.startswith("thread.") block was unreachable dead code, since a string cannot simultaneously equal "error" and start with "thread.".

Changes

• Moved error event handling (sse.event == "error") to a separate check before the thread.* event handling
• Applied fix to both sync (Stream.__stream__) and async (AsyncStream.__stream__) implementations
• Changed if to elif for the thread.* check to maintain proper control flow

Before

if sse.event and sse.event.startswith("thread."):
    # ...
    if sse.event == "error":  # ❌ Unreachable - never true
        raise APIError(...)

After

if sse.event == "error":  # ✅ Now reachable
    # ...
    raise APIError(...)
elif sse.event and sse.event.startswith("thread."):
    # ...

Test Plan

• Ruff check passes
• Existing tests pass

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