fix(oidc): don't mutate shared config on Facebook id_token login

[suvvvv]

What

In getProvider, the Facebook branch aliases the shared configuration and mutates it:

case p.Provider == FacebookProvider || p.Issuer == provider.IssuerFacebook:
    cfg = &config.External.Facebook // pointer INTO the process-wide *conf.GlobalConfiguration
    cfg.SkipNonceCheck = true        // writes shared state on every request

config is the long-lived *conf.GlobalConfiguration shared across all requests, so cfg aliases config.External.Facebook and this writes shared state on every Facebook id_token login. Concurrent Facebook logins — and any concurrent reader of config.External.Facebook — race on this write (go test -race territory). The written value is always true, so the practical harm is limited, but it is a real data race and an unexpected persistent mutation of shared config.

The custom: and default branches already avoid this by building a local conf.OAuthProviderConfiguration value.

Fix

Copy the provider config into a local value and set SkipNonceCheck on the copy, leaving the shared config untouched.

Testing

Added TestGetProviderFacebookDoesNotMutateSharedConfig: with Facebook disabled, getProvider returns before any OIDC discovery (no network), and the test asserts config.External.Facebook.SkipNonceCheck remains false after the call — it is true on the old aliasing code. go vet, gofmt, and the OIDC test suite pass.