feat(cli): add shell completion support

[Ahmed-AdelB]

Summary

This PR adds shell completion support to the OpenAI CLI, addressing issue #843.

Features

• Bash completion: Tab completion with compgen and bash-completion library
• Zsh completion: Native _arguments and _describe completion framework
• Fish completion: Declarative completions with conditions
• PowerShell completion: Register-ArgumentCompleter pattern

Usage

# Bash - add to ~/.bashrc
eval "$(openai completion bash)"

# Zsh - add to ~/.zshrc
eval "$(openai completion zsh)"

# Fish - save to completions directory
openai completion fish > ~/.config/fish/completions/openai.fish

# PowerShell - add to $PROFILE
. (openai completion powershell | Out-String)

Commands Supported

• Main commands: api, tools, completion
• API subcommands: chat.completions.create, models.list, models.retrieve, files.*, audio.*, images.*, embeddings.create, moderations.create
• Tools subcommands: fine_tunes.prepare_data
• Global options: -v, -k, -o, -t, --api-key, --organization, etc.

Implementation Notes

1. No external dependencies: Uses native shell completion primitives rather than libraries like argcomplete
2. Security: All completion values are hardcoded static strings - no dynamic content or user input processing
3. Minimal changes: Only adds one new file (_completion.py) and minimal modifications to _cli.py

Changes

• src/openai/cli/_completion.py (new): Shell completion script generator
• src/openai/cli/_cli.py: Added import and registration for completion subcommand

Test Plan

• Generated bash completion script and verified syntax
• Generated zsh completion script and verified syntax
• Generated fish completion script and verified syntax
• Generated PowerShell completion script and verified syntax
• Verified all API commands are included
• Verified all global options are included

Fixes #843

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Author: Ahmed Adel Bakr Alderai