[EXT-2180] OIDC: separate OIDC context cookie on navigation and background

[maximyurchuk]

Changelog entry

Updates OIDC proxy state handling so the callback can select the correct OIDC context cookie (navigation and background), preventing navigation redirects from being affected by AJAX auth challenges.

Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

• Separate OIDC context cookie for navigation and background.
• Add a functional test for mixed navigation and AJAX flows to make sure the callback redirects to the original page.
• After digest verification, the decoded state container must be valid JSON; malformed containers are now rejected explicitly.