fix(dashboard-auth): route password-capable providers to /login instead of /auth/login

[jon9314]

Reproduces and fixes the tracked BasicAuthProvider 500 on current main.

• Root cause: auto-sso middleware sends a single interactive provider to /auth/login, but password-capable providers implement no OAuth redirect flow.
• Fix: middleware now routes password-capable providers to /login so the form can render.
• Added regression coverage for redirect target, form rendering, and /auth/password-login returning ok.

Fixes #58166

[alt-glitch]

This was generated by AI during triage.

Duplicate of #54887 (earliest-open canonical fix for the dashboard password-only auth 500). Both patch _auto_sso_response in middleware.py with the same supports_password guard to avoid the OAuth-redirect flow; #54887 falls through with return None, this PR does an explicit redirect to /login — same code site, same mechanism. See also the broader superset PR #58171 (also fixes the /auth/login route).

[jon9314]

Closing as redundant — the dashboard auth redirect bug this targets was fully fixed on main by #61330 (merged as commit 4734071), which includes the supports_password guard plus the sibling routes.py/auth_login redirect site this PR addressed. Thanks!