fix(examples): update outdated shebangs and deprecated API usage

[edenfunf]

Summary

Several example scripts still reference poetry in their shebangs and use the legacy Completions API, which is no longer documented in the README. This PR brings them up to date:

• Shebangs: demo.py, async_demo.py, streaming.py, and video.py all used #!/usr/bin/env -S poetry run python. The project migrated to Rye and other examples (e.g. audio.py, speech_to_text.py) already use rye run python. Updated the four remaining files for consistency.

• Deprecated API: async_demo.py and streaming.py used the legacy Completions endpoint (client.completions.create with gpt-3.5-turbo-instruct). This endpoint isn't referenced in the README and gpt-3.5-turbo-instruct is one of the last models supporting it. Migrated both to the Chat Completions API (client.chat.completions.create).

• Model name: demo.py referenced gpt-4. Updated to gpt-4o to reflect current recommended usage.

Changes

File	Change
examples/demo.py	shebang poetry → rye, model gpt-4 → gpt-4o
examples/async_demo.py	shebang poetry → rye, completions.create → chat.completions.create
examples/streaming.py	shebang poetry → rye, completions.create → chat.completions.create
examples/video.py	shebang poetry → rye

Test plan

• ruff check passes on all modified files
• ruff format --check passes on all modified files
• Existing streaming tests (tests/test_streaming.py) pass
• import openai works correctly