Function completion 

pub fn completion(source: &str, offset: usize) -> Vec<CompletionItem>

Returns completions appropriate at offset (byte offset) in source.

Uses the parsed AST to determine context:

• Outside all declarations -> top-level keywords.
• Inside a datasource or generator block -> config key suggestions.
• Inside a model block:
  • After @ -> field attribute names.
  • After @@ -> model attribute names.
  • Otherwise -> scalar types, user-defined model/enum names, and common field attributes as a convenience.