gh-7: Add unit tests for pure configuration helpers.

[breadonb]

This PR adds unit tests for the dependency-free helper functions in manage.config.

Refs #7

Functions tested

• config_bool
• _expand_vars
• config_append
• config_dict_merge
• config_split
• config_split_append

Coverage for config.py increased from 17% to 30%. The full test suite increased from 415 to 456 tests, with all tests passing.

The tests satisfy the constraint from the issue by importing the functions directly and passing plain strings and dictionaries. They do not rely on a py command, the filesystem, the registry, _native, or any new dependency.

I deliberately scoped this PR to the dependency-free helpers. Tests for resolve_config, merge_config, and the load_* functions would be a natural follow-up.

Tested on Windows with Python 3.14.4 and pytest 9.1.1.

One behavior captured by the tests is that : is a delimiter for config_split, so "C:\\tools" splits into ["C", "\tools"]. The test test_config_split[windows_path] documents the current behavior rather than proposing a change.

[python-cla-bot]

All commit authors signed the Contributor License Agreement.