Make openai.rs generic to share logic with azureopenai.rs

[thisisharsh7]

Changes

Made openai::Client generic over config type to eliminate duplication with azureopenai.rs

Results

• Eliminates duplicated logic for embeddings, generation & json schema handling
• follows same pattern as litellm.rs and openrouter.rs

Fixes #1364

[georgeh0]

Please also fix the format check problem. You may want to install the pre-commit hook (https://cocoindex.io/docs/contributing/setup_dev_environment), and it'll automatically do a reformat.

[thisisharsh7]

got it will do thanks!