fix: Neo4j driver incompatible with Apache AGE connection URI

## Problem
The `Neo4jDatabase` class uses the Neo4j driver which expects `bolt://` or `neo4j://` URI schemes.
However, Apache AGE runs on PostgreSQL and uses `postgres://` URIs, causing a `ConfigurationError`
when running tests.

## Error
```
neo4j.exceptions.ConfigurationError: URI scheme 'postgres' is not supported.
Supported URI schemes are ['bolt', 'bolt+ssc', 'bolt+s', 'neo4j', 'neo4j+ssc', 'neo4j+s']
```
## Steps to reproduce
1. Set up Apache AGE with `postgres://localhost:5432`
2. Run `pytest tests/unit/`

## Possible solutions
- Use `apache_age_python` driver directly instead of Neo4j driver for AGE connections
- Or document the correct URI format for connecting Neo4j driver to AGE

Start with the Neo4jDatabase class and run pytest tests/unit/ to reproduce the ConfigurationError with an Apache AGE postgres:// URI. Compare the proposed apache_age_python driver approach with documenting a compatible URI format; done means the unit tests run successfully with Apache AGE and the chosen connection behavior is documented or supported.

Written by the indexing model from the issue text.