fix: remove duplicate accumulate_delta in assistants streaming

[bunlongheng]

Summary

lib/streaming/_assistants.py contained a private copy of accumulate_delta (lines 983-1041) that was byte-for-byte identical to the canonical version in lib/streaming/_deltas.py:6-64.

The chat streaming path already imports from _deltas (see lib/streaming/chat/_completions.py:25), but assistants streaming maintained its own duplicate - meaning any bugfix to one copy would silently miss the other.

Change

• Remove the 59-line duplicate from _assistants.py
• Import accumulate_delta from ._deltas (same as chat streaming already does)
• Remove the now-unused is_dict/is_list imports from _utils

Verification

ruff check src/openai/lib/streaming/_assistants.py  # all checks passed
pytest tests/lib/test_assistants.py tests/test_streaming.py  # 28 passed

No behavior change - both call sites (lines 910, 961) continue calling the same function, now through the shared import.