fix(dashboard): keep password auth on login form

[jeeves-assistant]

Summary

• Prevent password-only dashboard auth providers from being auto-routed through the OAuth /auth/login flow.
• Make direct /auth/login?provider=<password-provider> requests redirect back to /login instead of surfacing NotImplementedError as a 500.
• Adds regressions for both the unauthenticated HTML auto-SSO path and direct login URL path.

Test plan

• PYTHONPATH=. python -m pytest -q -o 'addopts=' tests/hermes_cli/test_dashboard_auth_password_login.py

[jeeves-assistant]

Closing this as a duplicate. After opening it, I found several existing open PRs for the same dashboard basic/password-auth 500 bug (for example #56666, #56886, #56627, #56306, #56639, #55988, #55133, #54887). Current origin/main still reproduces the issue, so the bug is real, but this PR should not be the artifact of record.

[jeeves-assistant]

Duplicate of existing dashboard basic/password-auth 500 PRs; closing to avoid adding another competing artifact.