fix: fixed nested aggregations deserialization

[mdashti]

What

Fixed deserialization of nested aggregations when the aggs field is embedded inside an aggregation variant's configuration object.

Why

Previously, aggregation requests with nested aggs like {"terms": {"field": "category", "aggs": {...}}} failed to deserialize correctly. The nested aggregations were being ignored because the deserializer expected aggs at the top level, not inside the aggregation variant's config.

How

Modified the TryFrom<AggregationForDeserialization> implementation to:

1. Scan the aggregation config object for nested aggs fields
2. Extract and parse them as sub-aggregations before deserializing the main variant
3. Assign the extracted nested aggregations to the sub_aggregation field

The implementation uses a functional approach to find and extract the first nested aggs field from the aggregation configuration.

Tests

Added three test cases covering:

• Basic nested aggregations (terms → terms)
• Triple-nested aggregations (terms → terms → terms)
• Nested aggregations with metric sub-aggregations (terms → avg/max)