LG-12070: Maintain incoming request for concurrent session logout

[aduth]

🎫 Ticket

LG-12070

Related issue: #9407

🛠 Summary of changes

Updates OIDC and SAML controllers to set redirect URL ahead of concurrent session logout, to repeat the authentication request as redirect destination for the logout. This resolves an issue where authentication request details would otherwise be lost during the concurrent session logout.

The solution here is essentially the same as in LG-11777 (#9842), except applied to the authentication endpoints in addition to the logout endpoints handled there.

📜 Testing Plan

Prerequisite: Have both OIDC and SAML sample applications running separate from the IdP

1. Start at http://localhost:9292 (OIDC) or http://localhost:4567 (SAML)
2. Click "Sign in"
3. Complete sign in until returned to sample application
4. In a different browser or private browsing window, repeat Steps 1-3, using the same sample application
5. In your original browser, repeat Steps 1-2, using the other of the two links
6. Note that "X is using Login.gov..." text reflects the name of the sample application you came from in Step 5

[aduth]

This link_identity code wasn't strictly necessary so was removed to avoid excessive duplication in the new, similar test cases being added.