Fix Microsoft OIDC authentication failure

[gearnode]

Summary

• Set AuthStyle: oauth2.AuthStyleInParams on the Microsoft OAuth2 endpoint
• Without this, the oauth2 library auto-detects the auth style by first trying Basic auth (header), which Microsoft rejects — and since PKCE authorization codes are single-use, the retry with credentials in the POST body fails with the already-consumed code
• This matches the official golang.org/x/oauth2/microsoft package behavior

Test plan

• Sign in with Microsoft OIDC and verify authentication succeeds
• Sign in with Google OIDC and verify it still works

---

Summary by cubic

Fix Microsoft OIDC sign-in by sending client credentials in the token request body. This prevents PKCE code reuse failures caused by oauth2 auto-detect trying Basic auth.

• Bug Fixes
  • Set AuthStyle: oauth2.AuthStyleInParams for the Microsoft OAuth2 endpoint to match golang.org/x/oauth2/microsoft.

^{Written for commit 3879641. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file