fix(examples): refresh chat streaming examples (#2974)

## 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](https://rye.astral.sh/) 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

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

Co-authored-by: Marcus Wood <marcuswood@openai.com>