feat(oidc): add discovery_url override and claims_mapping for OIDC pr…

[montaro]

…oviders [WIP]

[github-actions]

👋 Hi @montaro! Thanks for your contribution to this project.

It looks like one or more of your commits are missing a DCO (Developer Certificate of Origin) sign-off. The DCO is a simple way for you to certify that you have the right to submit this code under the project's license.

How to fix this:

# For future commits, use the -s flag
git commit -s -m "Your commit message"

# To sign off on existing commits in this PR
git rebase HEAD~$(git rev-list --count origin/v2..HEAD) --signoff
git push --force-with-lease

The -s flag adds this line to your commit message:
Signed-off-by: Your Name <your.email@example.com>

📋 View the failing DCO check for more details

For more information about the DCO, visit: https://developercertificate.org/

[github-actions]

Verdict: approve

Clean, well-tested auth change. The discovery_url override correctly uses go-oidc's InsecureIssuerURLContext while pinning token verification to issuer_url, and claims_mapping handles arrays, missing values, and non-strings properly. No code issues found. (Note: the PR is still marked [WIP] and the DCO check is failing — commits need sign-offs.)

🤖 Automated review by the Flipt PR review agent.

[codecov]

Codecov Report

❌ Patch coverage is 92.15686% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.45%. Comparing base (b7aec53) to head (2b76f70).
⚠️ Report is 12 commits behind head on v2.

Files with missing lines	Patch %	Lines
internal/config/authentication.go	71.42%	1 Missing and 1 partial ⚠️
internal/server/authn/method/oidc/server.go	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #6233      +/-   ##
==========================================
+ Coverage   62.41%   62.45%   +0.04%     
==========================================
  Files         145      145              
  Lines       14867    14910      +43     
==========================================
+ Hits         9279     9312      +33     
- Misses       4828     4836       +8     
- Partials      760      762       +2     

Flag	Coverage Δ
integrationtests	33.28% <0.00%> (-0.16%)	⬇️
unittests	54.09% <92.15%> (+0.10%)	⬆️

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