Simplify authserver's upstream provider interface

[jhrozek]

Follow-up to an earlier discussion in a previous PR: #3580 (comment)

Combine ExchangeCode and ResolveIdentity into atomic ExchangeCodeForIdentity

Merge the two-step exchange+identity flow into a single interface method to prevent OIDC nonce validation from being accidentally skipped. This ensures replay protection is always enforced when exchanging authorization codes with OIDC providers.

The OAuth2Provider interface shrinks from 6 to 4 methods by removing ExchangeCode, ResolveIdentity, and FetchUserInfo (now package-private). The UserInfo struct is also made private and moved to oauth2.go.

Large PR Justification

• renames and simplifies an interface implemented by 2 modules along with tests and mocks. There are no logical changes here.

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[github-actions]

✅ Large PR justification has been provided. The size review has been dismissed and this PR can now proceed with normal review.

[codecov]

Codecov Report

❌ Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.96%. Comparing base (788057b) to head (a7b2fb6).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/authserver/upstream/oauth2.go	87.50%	1 Missing and 1 partial ⚠️
pkg/authserver/upstream/oidc.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3638      +/-   ##
==========================================
+ Coverage   65.75%   65.96%   +0.20%     
==========================================
  Files         411      413       +2     
  Lines       40805    41062     +257     
==========================================
+ Hits        26833    27086     +253     
+ Misses      11882    11881       -1     
- Partials     2090     2095       +5     

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