Problem

#1300 introduced immediate tool_called event emission during streaming, but it was emitting events on ResponseOutputItemAddedEvent which has empty arguments instead of ResponseOutputItemDoneEvent which has complete arguments.

The fix

Simple one-line change:

# Before (broken)
if isinstance(event, ResponseOutputItemAddedEvent):

# After (fixed) 
if isinstance(event, ResponseOutputItemDoneEvent):

Now tool calls are emitted when they're complete with full arguments instead of when they start with empty arguments.

New tests

• Comprehensive regression tests to make sure this doesn't happen again.

Fixes #1629