Function resolve_fields 

pub fn resolve_fields(
    params: &Map<String, Value>,
    field_values: &BTreeMap<String, Value>,
) -> Map<String, Value>

Substitute {{fields.*}} placeholders in all string values within params.

Walks the params map and replaces occurrences of {{fields.X}} with the corresponding value from field_values. Non-string values pass through unchanged. Supports substitution inside array elements and nested strings containing multiple placeholders.