refactor(copilotchat-nvim): improve picker selection logic and error handling

[tku137]

This is an attempt to further improve on #1333.

First, a separate function to check for the currently active pickers name or nil if it has no CopilotChat.integrations module. Since I did not find a better solution to check if snacks.nvim picker is actually used as the picker (solely checking for snacks.nvim availability does not mean that it's picker is actually configured and used, contrary to telescope or fzf-lua), I tried finding something to find out, which is up for debate. Second, the create_mapping() function now checks for availability and compatibility of the active picker with CopilotChat integrations and gracefully warn users if it's not.

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).

[azdanov]

Looks good, but I don't understand most of it (low vim skills on my side) 😄

[mehalter]

Looks great! Thanks so much!