Function get_function_completions 

pub fn get_function_completions(
    ctx: &FunctionCompletionContext,
) -> Vec<CompletionItem>

Gets function completions filtered and scored by context.

This function returns SQL functions as completion items, filtered by prefix and with scoring adjustments based on the current context:

• Aggregate functions are boosted when GROUP BY is present
• Aggregate functions are penalized in WHERE clause
• Window functions are boosted in OVER/WINDOW context

Performance

Uses a pre-computed cache of function items. On each call, we:

1. Filter the cache by prefix (if provided)
2. Clone matching items
3. Apply context-based scoring

This avoids re-computing signatures and format strings on every request.