test: add config validation tests

[haosenwang1018]

Adds configuration validation tests (#114).

All 6 tests passed:

• Default config creation ✓
• Default provider ✓
• Default model ✓
• Default Milvus URI ✓
• Default collection ✓
• Chunking defaults ✓

Part of #114

[zc277584121]

The config validation idea has some value, but a few things:

• Check if these overlap with the existing tests/test_config.py.
• test_config_default_model asserts against DEFAULT_MODELS["openai"] directly rather than the config object's resolved model — fix the assertion target.
• Remove unused import pytest and the duplicate test_cli_help.py.