tests: add offline API pytest foundation

[tokenledger]

Summary

• add the required hello-world smoke test
• add positive and invalid-value coverage for API configuration parsing
• add pytest src-layout configuration and document the offline unit-test command
• correct the unit fixture dev-mode variable and prevent object-store startup so the suite stays offline

Existing application and dependency test modules are unchanged.

Validation

• API unit suite: 8 passed
• ruff check src/ndif/services/api/tests/unit/test_config.py src/ndif/services/api/tests/unit/test_smoke.py
• ruff format --check src/ndif/services/api/tests/unit/test_config.py src/ndif/services/api/tests/unit/test_smoke.py
• git diff --check

Closes #172