feat(oidc): support configurable token signing algorithms

[gzxu]

Summary

Add an algorithms field to the OIDC provider configuration so Flipt can verify ID tokens signed with algorithms other than RS256.

closes #5361.

Changes

• Update config schema to allow authentication.methods.oidc.providers.<provider>.algorithms (default ["RS256"])
• Plumb configured algorithms into OIDC provider setup
• Update OIDC server tests to include algorithms config

Test plan

• Run existing unit tests for OIDC auth method (updated tests included)

[dosubot]

Related Documentation

Checked 4 published document(s) in 1 knowledge base(s). No updates required.

^{How did I do? Any feedback?}  

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.54%. Comparing base (8ac6346) to head (4076ee4).
⚠️ Report is 1 commits behind head on v2.

Files with missing lines	Patch %	Lines
internal/server/authn/method/oidc/server.go	66.66%	1 Missing and 1 partial ⚠️
internal/config/authentication.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #5362      +/-   ##
==========================================
+ Coverage   59.97%   60.54%   +0.56%     
==========================================
  Files         138      138              
  Lines       13578    13584       +6     
==========================================
+ Hits         8144     8225      +81     
+ Misses       4735     4662      -73     
+ Partials      699      697       -2     

Flag	Coverage Δ
integrationtests	34.64% <0.00%> (+1.51%)	⬆️
unittests	51.95% <57.14%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[erka]

Please update config/flipt.schema.json with a new option to improve in editor support. And another PR with oidc also missing it.

[erka]

@gzxu thank you for the PR.

[gzxu]

Thanks @erka for reviewing! Quick question: Which command can I use to auto-generate config/flipt.schema.json from config/flipt.schema.cue?

[erka]

You have to do it manually. Cue didn't support it in the past. You could ask llm to do it as alternative. They usually do a good job in this case.

[gzxu]

Ah you're right, LLM did a good job. Thanks!

[markphelps]

looks great! ty for your contribution @gzxu !!