DB Tests use common driver name expansion with application code

[beforan]

Pull Request Contents

🦋 Bug Fix

Description

This PR refactors the db drivername expansion into a function, and then ensures the function is used by the application code and database integration tests.

Related Issues or other material

• This PR relates to one or more issues, detailed below

• Related Issue #
• Closes Tests fail using documented example configuration #17

Screenshots, example outputs/behaviour etc.

> export DATASOURCE_DB_DRIVERNAME=postgresql
uv run pytest
================================================ test session starts =================================================
platform linux -- Python 3.13.1, pytest-8.3.4, pluggy-1.5.0
rootdir: /home/beforan/src/hutch-bunny
configfile: pyproject.toml
collected 9 items                                                                                                    

tests/test_demographics_distribution_query.py .....                                                            [ 55%]
tests/test_return.py ....                                                                                      [100%]

================================================= 9 passed in 1.96s ==================================================

✅ Added/updated tests?

• This PR contains relevant tests
  • Or doesn't need to per the below explanation

• I've completed all actions and tasks detailed in the PR Template