Phase 2, Task 2.1. Self-contained fake IDP — start_login redirects
straight back to {redirect_uri}?code=stub_code&state=<s> so tests can
walk the OAuth round trip in-process. Tokens are HMAC-signed JSON blobs
(not real JWTs) — enough structure for verify_session to detect tamper
and expiry without pulling in pyjwt.
Lives in tests/ only — never registered as a real plugin. Phase 3's
end-to-end tests import StubAuthProvider directly.
Convention: exp <= now counts as expired (TTL=0 means born-expired)
— matches what Phase 6's silent-refresh test will need.
