SimpleAuthManager: preserve deep-link next URL on first login

[GayathriSrividya]

Fixes #67476

When unauthenticated users opened a deep link, some login paths called get_url_login(next_url=...). SimpleAuthManager ignored this kwarg, so it always redirected to /auth/login or /auth/token/login without preserving next, which caused users to land on the homepage after their first login.

This change makes SimpleAuthManager.get_url_login honor the optional next_url argument by appending a URL-encoded next query parameter for both regular and all-admin login endpoints.

Added unit tests for:

• Regular login with next_url
• All-admin login with next_url

Validation:

• /opt/homebrew/bin/ruff format + check --fix passed
• Local pytest run failed in this workspace due to a missing test dependency: tests_common

[vincbeck]

Fixed in #67483?

[GayathriSrividya]

Good question. I checked this against #67483, and there is still a gap this PR covers.

What #67483 fixed:

• It updated the /auth/token/login route to honor a next query param if present.

What it did not change:

• SimpleAuthManager.get_url_login still ignored next_url and always returned /auth/login or /auth/token/login without appending next.

Why this still matters:

• FAB call sites already pass next_url into get_url_login:
  • providers/fab/src/airflow/providers/fab/www/auth.py
  • providers/fab/src/airflow/providers/fab/www/extensions/init_appbuilder.py
• Because SimpleAuthManager discarded next_url, deep-link context was lost before reaching /auth/token/login.

So #67483 handles consumption of next, while this PR handles propagation of next into the login URL generation path.