simple-engine: keep tool chat on the streaming execution path

[Thump604]

Supersedes #222 with the same functional change rebased on current main.

Why this PR exists:

• I rebased the original branch locally and validated it cleanly.
• Direct maintainer push back to the original computor-org fork is not available from this auth context, so the original PR could not be updated in place.

Scope:

• route tool-enabled non-stream SimpleEngine.chat() through the streaming chat path
• preserve streamed token ids and final finish_reason/completion accounting
• carry the regression coverage in tests/test_simple_engine.py on top of current main

Local validation:

• python -m py_compile vllm_mlx/engine/simple.py tests/test_simple_engine.py
• python -m black --check --fast vllm_mlx/engine/simple.py tests/test_simple_engine.py
• pytest -q tests/test_simple_engine.py

Authorship note: this restack preserves the original implementation from #222; the only local adjustment was aligning the regression assertion with current main's cleaned tool-call text behavior.