fix(streaming): move unreachable error event check out of thread. branch

[MaxwellCalkin]

Summary

Fixes #2796.

The sse.event == "error" check in both Stream.__stream__ and AsyncStream.__stream__ is dead code — it's nested inside if sse.event.startswith("thread."), but "error" never starts with "thread.".

Root cause

This was introduced in commit 48188cc when the branch logic was refactored from checking for response.*/transcript.* events first to checking for thread.* events first. The sse.event == "error" check was in the else branch (non-response.* events) before the refactor, but was accidentally placed in the thread.* branch after it. The subsequent indentation fix in abc2596 preserved the mistake.

Before 48188cc (correct):

if sse.event is None or (sse.event.startswith("response.") or ...):
    # generic error check
    if is_mapping(data) and data.get("error"): ...
else:
    # event-specific error check — reachable for event="error"
    if sse.event == "error" and ...: ...

After 48188cc (broken):

if sse.event and sse.event.startswith("thread."):
    # error check is now unreachable — "error" != "thread.*"
    if sse.event == "error" and ...: ...
else:
    if is_mapping(data) and data.get("error"): ...

Fix

Move the error event handling into its own elif sse.event == "error": branch between the thread.* check and the else fallback, so it can actually be reached when the server sends an SSE with event: error.

if sse.event and sse.event.startswith("thread."):
    ...
elif sse.event == "error":
    # now reachable
    ...
else:
    ...

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

Test plan

• Verify the elif branch is reached for event: error SSEs
• Verify thread.* events still handled correctly (no behavior change)
• Verify other events still fall through to else branch

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve event:error items when payload lacks error key

This new elif sse.event == "error" branch no longer yields anything when the payload does not contain a top-level error field, so event:error SSEs can be silently dropped. In Assistants streaming, ErrorEvent is modeled as { "event": "error", "data": ErrorObject } (not {"error": ...}), so this path will skip the APIError raise and also skip process_data, meaning consumers never receive the event; the same regression is duplicated in AsyncStream.__stream__.

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