fix(oidc): return M_UNRECOGNIZED when OIDC server is not configured

[f3nr1l]

Summary

When OAuth/OIDC is not configured, /_matrix/client/v1/auth_metadata and /_matrix/client/v1/auth_issuer currently propagate the services.oauth.get_server() error as-is, which surfaces to clients as a generic M_NOT_FOUND.

Element Web (>= 1.12, via matrix-js-sdk's buildValidatedConfigFromDiscovery) only treats MSC2965 discovery as "not supported" when it receives 404 + M_UNRECOGNIZED. Any other error — including 404 + M_NOT_FOUND — is treated as a fatal misconfiguration and the login screen shows " is misconfigured".

This PR makes both endpoints return 404 + M_UNRECOGNIZED ("OIDC server not configured") when no OAuth server is configured, matching the behavior of the router's default not_found handler. Password-only homeservers then work out of the box with recent Element Web builds.

Reproduction

1. Deploy Tuwunel with login_with_password = true and no [[global.identity_provider]] block.
2. Open the latest Element Web, point it at that homeserver.
3. Before the patch: login screen stuck on " is misconfigured".
4. After the patch: login screen shows the password form as expected.

Changes

• src/api/oidc/auth_metadata.rs — intercept get_server() error, return Error::Request(ErrorKind::Unrecognized, ..., StatusCode::NOT_FOUND).
• src/api/oidc/auth_issuer.rs — same fix.

Tested on

Built from this branch, deployed on matrix.altanet.fr (Tuwunel 1.6.0 + this patch). Element Web login flow, password auth, federation all nominal. Running for several days without regression.

Notes

• No change to the happy path when OIDC is configured.
• No new dependencies.
• Reference for the expected behavior: the default not_found handler in src/router/router.rs already returns M_UNRECOGNIZED for unknown routes.