client/python: add OpenAI-compatible REST API samples

[retryoos]

🛠 Summary

JIRA/Issue if applicable: N/A (small Python client samples addition)

This PR adds OpenAI-compatible REST API Python samples under client/python/openai-api/samples/ and links them from client/python/README.md.

Added:

• http_list_models.py for GET /v3/models
• http_chat_completions.py for unary POST /v3/chat/completions
• http_chat_completions_stream.py for streaming POST /v3/chat/completions (stream: true, SSE parsing)
• README.md with setup, usage, and examples
• requirements.txt (stdlib-only note)

Updated:

• client/python/README.md with a link to the new OpenAI API samples section

Notes:

• Local validation done with python3 -m py_compile and --help checks for all scripts.
• Runtime error path was validated without a running OVMS instance (clear connection-refused behavior).
• End-to-end runtime test depends on OVMS startup/model load.

🧪 Checklist

• Unit tests added.
• The documentation updated.
• Change follows security best practices.