Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

services/auth/source/oauth2/providers_test.go

@@ -19,11 +19,11 @@ func (p *fakeProvider) Name() string {
 func (p *fakeProvider) SetName(name string) {}
 
 func (p *fakeProvider) BeginAuth(state string) (goth.Session, error) {
-	return nil, nil
+	return nil, nil //nolint:nilnil // the auth method is not applicable
 }
 
 func (p *fakeProvider) UnmarshalSession(string) (goth.Session, error) {
-	return nil, nil
+	return nil, nil //nolint:nilnil // the auth method is not applicable
 }
 
 func (p *fakeProvider) FetchUser(goth.Session) (goth.User, error) {