Linting/Formatting of main app files

[Snuffy2]

This pull request introduces improvements to documentation, type annotations, and error handling, as well as code cleanup for better maintainability and clarity. The most significant changes are grouped below.

Documentation and Installation Improvements:

• Updated README.md to clarify installation instructions for image generation/editing, now supporting both manual installation and an extras-based install (mlx-openai-server[image-generation]). Added instructions for enhanced caching support via an enhanced-caching extra. [1] [2] [3] [4] [5]

Type Annotations and Code Cleanup:

• Added or improved type annotations for functions and classes across app/cli.py, app/config.py, app/main.py, and app/server.py for better code clarity and static analysis. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

Error Handling Improvements:

• Enhanced error logging to include exception types and stack traces, using logger.exception for more informative diagnostics during server startup, handler initialization, and shutdown. [1] [2]

General Codebase Cleanup:

• Removed unused global variables and legacy comments, and refactored function docstrings for clarity and consistency. [1] [2] [3]

Middleware and Server Setup Enhancements:

• Improved the HTTP middleware in app/server.py with explicit type annotations and more detailed docstrings for request handling and process time measurement.

[Snuffy2]

@cubist38 any chance of getting these Linting PRs reviewed and merged? I've been rebasing them to keep them ready to merge.