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/oauth2.go

@@ -156,12 +156,12 @@ func (o *OAuth2) Verify(req *http.Request, w http.ResponseWriter, store DataStor
 	detector := newAuthPathDetector(req)
 	if !detector.isAPIPath() && !detector.isAttachmentDownload() && !detector.isAuthenticatedTokenRequest() &&
 		!detector.isGitRawOrAttachPath() && !detector.isArchivePath() {
-		return nil, nil
+		return nil, nil //nolint:nilnil // the auth method is not applicable
 	}
 
 	token, ok := parseToken(req)
 	if !ok {
-		return nil, nil
+		return nil, nil //nolint:nilnil // the auth method is not applicable
 	}
 
 	user, err := o.userFromToken(req.Context(), token, store)