Add test for vendor_part_id=None to achieve 100% coverage

Author: dsfaccini

tests/test_parts_manager.py

@@ -338,6 +338,27 @@ def test_handle_text_deltas_with_split_tags_and_interleaved_content():
     )
 
 
+def test_handle_text_deltas_with_split_tags_no_vendor_id():
+    """Test that potential tag handling works when vendor_part_id is None."""
+    manager = ModelResponsePartsManager()
+    thinking_tags = ('<think>', '</think>')
+
+    # When vendor_part_id is None and content could be start of tag, no event is returned
+    # but we can't buffer (no vendor_part_id to store under)
+    event = manager.handle_text_delta(vendor_part_id=None, content='<thi', thinking_tags=thinking_tags)
+    # Returns None because it might be a tag start
+    assert event is None
+    # But no part is created yet (can't buffer without vendor_part_id)
+    assert manager.get_parts() == snapshot([])
+
+    # Next chunk shows it's not a tag, process as normal
+    event = manager.handle_text_delta(vendor_part_id=None, content='s is text', thinking_tags=thinking_tags)
+    assert event == snapshot(
+        PartStartEvent(index=0, part=TextPart(content='s is text', part_kind='text'), event_kind='part_start')
+    )
+    assert manager.get_parts() == snapshot([TextPart(content='s is text', part_kind='text')])
+
+
 def test_handle_tool_call_deltas():
     manager = ModelResponsePartsManager()