feat(command): add model command to get/set the model

[doums]

Hi, currently it's not possible to get what is the current model in-use (which is quite opaque). This PR add new commands to get/set/select the model at runtime and view the available model(s).
For now, there is only one model available which is GPT-4.1 Copilot.

Catches up with what has been done in copilot.vim side github/copilot.vim#139 (comment)

Summary

Add a new :Copilot model command that allows users to view and switch between available Copilot completion models at runtime.

Changes

• New model subcommand with the following actions:
  • select - Interactive model picker using vim.ui.select*
  • list - Display all available completion models
  • get - Show the currently active model
  • set <model-id> - Set a specific model by ID
• Runtime model override - Selected model is applied immediately without requiring a restart, by notifying the LSP server of configuration changes
• Model validation on startup - If a copilot_model is configured, it's validated against available models and a warning is shown if invalid
• API addition - New copilot/models request to fetch available models from the Copilot server

For select if there is only one available model (mostly the case) and the current in-use does not match, it is automatically selected.

Files Changed

File	Description
lua/copilot/model.lua	New module handling model selection, listing, and validation
lua/copilot/api/init.lua	Added get_models API function
lua/copilot/client/config.lua	Validate configured model on client startup
lua/copilot/client/utils.lua	Use model module for getting current model (supports runtime override)
plugin/copilot.lua	Register model command with completions