Add baseline test harness and CI

[jsachs]

Summary

This PR adds the first automated test foundation for the Python client:

• seven offline unittest cases covering configuration, the password-token happy path, request construction, JSON/text responses, and Pydantic alias serialization;
• reusable fake response and recording-session helpers for follow-up regression tests;
• GitHub Actions coverage for Python 3.10 through 3.13;
• a corrected minimum Python version of 3.10, matching syntax already used by the client; and
• a documented local test command.

The tests invoke the real Config, _update_auth, and _call implementations. Only the external HTTP session is replaced, so the suite is deterministic and requires no credentials or network access. A mutation check confirmed that incorrect refresh-token storage and an incorrect Bearer header are both detected.

Scope and limitations

This is intentionally a baseline harness, not a complete OpenAPI compliance suite. It does not yet cover every endpoint, client-credentials and refresh flows, MFA, error handling, generated NERIS models, response schemas, or live API behavior. Those cases can be added safely after the focused fixes below land; OpenAPI drift and model-generation checks are planned as follow-up work.

Bugs identified during the test audit

The audit exposed several existing production issues, kept out of this foundation PR so each fix remains independently reviewable:

Finding	Follow-up PR
Generated enums are not consistently serialized to their API values	#35
Unit creation and patch routes are incorrect	#36
Explicit configuration and user-agent values can be overridden incorrectly	#37
HTTP session and authentication state are shared between client instances	#38
JSON string payloads can be double-encoded	#39
Documented token variants, errors, and missing refresh tokens are mishandled	#43 (supersedes #40)
API HTTP errors are swallowed and returned instead of raised	#42 (breaking change)
Package metadata incorrectly advertises Python 3.8 support	fixed here; also isolated in #41

Verification

python -m unittest discover --start-directory tests --verbose

Ran 7 tests
OK