Facebook and some other OAuth providers issue authorization codes that
exceed 512 characters. Pydantic rejected these with 422 string_too_long.
The OAuth spec defines no maximum code length; 2048 aligns with common
provider limits.
Also adds three integration tests to verify 512-char and 2048-char codes
are accepted while 2049-char codes are correctly rejected.
PyJWT compares the iss claim against discovery_issuer with an exact string
match. Authentik (and similar providers) include a trailing slash in the JWT
iss claim while the discovery document issuer may omit it, or vice-versa.
Disable PyJWT built-in issuer validation and compare both sides after
rstrip('/') to make the check slash-agnostic.
Adds a regression test that verifies a login succeeds when the provider is
configured without a trailing slash but the JWT iss claim carries one.
maziggy