fix(ui): tolerate entra id RFC8414 deviation

[krancour]

MS Entra ID (formerly Azure AD) implements OIDC and PKCE, but deviates from RFC8414 (OAuth 2.0 Authorization Server Metadata) by failing to include the code_challenge_method in its metadata. This can create the impression that it does not support PKCE.

Up to now, this has been the cause of a (previously unknown) incompatibility with our UI.

This PR adds an explicit exception to the expectation that code_challenge_method exists and includes the value "S256" for Entra ID only.

This is safe, as support for S256 is documented here: https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-auth-code-flow

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.47%. Comparing base (5b27901) to head (062de60).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3359   +/-   ##
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  Coverage   52.47%   52.47%           
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  Files         291      291           
  Lines       26617    26617           
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  Hits        13968    13968           
  Misses      11886    11886           
  Partials      763      763           

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[akuitybot]

Successfully created backport PR for release-1.2:

• chore(backport release-1.2): fix(ui): tolerate entra id RFC8414 deviation #3362