Implement per-condition-set aggregation in Rust flag evaluator

Part of #46288

Context

The Rust flag evaluation service needs to support per-condition-set aggregation and mixed person + group property filters.

Requirements

1. Read aggregation_group_type_index from each condition set (not flag-level)
2. Allow conditions to contain both person and group property filters
3. Evaluate group properties against provided group_properties context
4. Use correct hash key based on condition set's aggregation:
   • None → hash distinct_id
   • 0-4 → hash groups[index]

Evaluation Logic

for condition_set in flag.conditions {
    if !matches_filters(&condition_set.properties, &person_props, &group_props) {
        continue;
    }
    
    let hash_key = match condition_set.aggregation_group_type_index {
        None => distinct_id.clone(),
        Some(idx) => groups.get(idx).cloned().unwrap_or_default(),
    };
    
    if hash_percentage(&hash_key, &flag.key) < condition_set.rollout_percentage {
        return true;
    }
}
return false;

Edge Cases

• If condition set has group filters but group context not provided → return false for that condition set (strict mode)
• If condition set aggregates on a group type but that group key not provided → skip to next condition set

Start in the Rust flag evaluation service and locate the condition-set evaluation path. Verify per-condition-set aggregation, mixed person and group filters, group_properties handling, and the specified missing-context edge cases; done means each requirement and edge case behaves as described.