Bugfix: Wrogn template is used when two different model families used for chat and autocomplete

[ldeninski]

This PR:

• Changes the feature argument in AiClient.getModel to be a enum (or a valid string in it)
• Removes the default argument value to avoid future bugs
• Adds some usefull typing utils
• Fixes the bug in AutoCompleteProvider.provideInlineCompletionItems where AIClient.getModel is called without an argument causing it to default to the chat model thus breaking autocomplete when the model families for chat and autocomplete are different