fix(streaming): Move error event handling outside thread.* condition

[jay-dhamale]

Summary

Fixes #2796 - Removes dead code in streaming error handling

Problem

The error handling logic for sse.event == "error" was incorrectly nested inside the if sse.event.startswith("thread.") condition at:

• Lines 67-79 in Stream.__stream__ method
• Lines 170-182 in AsyncStream.__stream__ method

This made the error handling code logically unreachable (dead code) because a string cannot simultaneously:

• Equal "error"
• Start with "thread."

Root Cause

Regression introduced in commit abc25966 which fixed indentation but accidentally moved the error handler from an else branch into the thread.* condition block.

Solution

Restructured the conditional logic to:

1. Check for sse.event == "error" FIRST (before any event type routing)
2. Handle thread.* events with their special data structure format
3. Handle all other events in the else block

This ensures error events are properly caught and raise APIError with the appropriate error message.

Changes

• Modified src/openai/_streaming.py:
  • Extracted error event handling to top-level if statement
  • Used independent if statements (not elif) for clear logic flow
  • Applied identical fix to both Stream and AsyncStream classes

Testing

• ✅ All existing tests pass (20/20 in test_streaming.py)
• ✅ Linting passes (ruff check .)
• ✅ Formatting passes (ruff format)
• ✅ No regressions in streaming functionality
• ✅ Module imports correctly

Test Plan

• Run full test suite: All streaming tests pass
• Verify both sync and async streams handle errors correctly
• No breaking changes - the buggy code path was unreachable