feat: Enhance SAML provider name resolution & update redirect handling

[bra-i-am]

Description

Fixes an issue where the TPA errors during the "link account" flow from the Account MFE were skipped, leaving the user with no feedback.

Root cause

When a SAML authentication error occurs (e.g. AuthAlreadyAssociated — the IdP account is already
linked to a different platform account), ExceptionMiddleware.get_redirect_uri() returned
/account/settings. That path is registered as a plain
django.views.generic.base.RedirectView pointing directly to ACCOUNT_MICROFRONTEND_URL. This
redirect is stateless — it does not read or forward Django messages, nor does it preserve query
parameters — so any error context was lost before the MFE ever loaded.

What changed

common/djangoapps/third_party_auth/middleware.py

• Added _get_saml_provider_name(request): reads the RelayState field from the SAML POST body
  (which contains the IdP slug as {"idp": "<slug>", ...}), looks up the corresponding provider in
  the TPA registry, and returns its human-readable display name (e.g. Cartão de Cidadão). Falls
  back to None gracefully on any parsing or lookup failure.

• Extended ExceptionMiddleware.get_redirect_uri(): when auth_entry == account_settings and the
  exception is a SocialAuthBaseException, the method now bypasses /account/settings and builds
  the Account MFE URL directly with ?duplicate_provider=<name>, using urllib.parse.quote() to
  encode the name as %20-spaced (not +-spaced) so the MFE renders it correctly. Falls back to
  the backend name (tpa-saml, OAuth provider name, etc.) if the SAML provider name cannot be
  resolved.

The Account MFE (frontend-app-account) already reads ?duplicate_provider= in
AccountSettingsPage.jsx and renders a danger alert — no frontend changes were required.

Impact

• Learners: The error message "The [Provider Name] account you selected is already linked to
  another [Site Name] account." is now correctly displayed in the Account MFE after a failed link
  attempt.
• Operators / Developers: No configuration changes required. The fix activates automatically for
  any auth_entry=account_settings TPA error.

Screencast

Screencast.from.01-04-26.09.03.33.webm

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Testing instructions

*This change was tested by using SimpleSAMLphp

1. Configure two platform accounts (user_a@example.com, user_b@example.com) and a SAML IdP.
2. Link the SAML identity to user_a via the normal login flow.
3. Log in as user_b.
4. Navigate to Account MFE → Linked Accounts.
5. Click "Sign in with <Provider Name>" for the same SAML IdP.
6. Complete authentication with the same identity on the IdP side.
7. Expected: Account MFE reloads and shows a red alert:

   "The [Provider Display Name] account you selected is already linked to another [Site Name]
   account."

[openedx-webhooks]

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