hierarchical country dimension handling

### StatGPT Backend version

0.1.0

### What is the problem this feature will solve?

assume the following hierarchy: `country (root) -> region -> state -> county -> city`

the feature should allow for better support of various queries to hierarchical country dimension:

- "give me X for region Y"
- "give me X for all states in region Y"

### What is the feature you are proposing to solve the problem?

design is to be discussed. but likely components are:

- identifier of required hierarchy levels
- `all_values` term added to each hierarchy level
- post-processing, to remove any hallucinations. for example if "state" level is specified, all non-state level terms selected should be removed from query

### What alternatives have you considered?

_No response_

No files, tests, or entry points are named. Start by mapping how hierarchical country dimensions and query levels are currently represented, then clarify the design for required hierarchy levels, `all_values`, and hallucination post-processing. Done means both region-level and all-states-in-region queries are supported with non-state terms removed when a state level is selected.

Written by the indexing model from the issue text.