Reduce ID token verification errors to a fixed category

go-oidc's RemoteKeySet puts the JWKS endpoint's response body in its error
for non-200 and malformed 200 responses; Verify wrapped it and the callback
logged it. verifyError now keeps only a fixed category, plus the expiry for
an expired token. A test with a sentinel JWKS body checks it reaches
neither the returned error, the response nor the logs.

Addresses feature-review finding F03-03 on PR #24.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/auth/auth.go

@@ -353,7 +353,7 @@ func (a *Auth) verify(ctx context.Context, code string, state loginState) (Claim
 	}
 	idToken, err := a.verifier.Verify(ctx, raw)
 	if err != nil {
-		return Claims{}, fmt.Errorf("verify ID token: %w", err)
+		return Claims{}, verifyError(err)
 	}
 	if subtle.ConstantTimeCompare([]byte(idToken.Nonce), []byte(state.Nonce)) != 1 {
 		return Claims{}, errors.New("ID token nonce mismatch")
@@ -404,6 +404,17 @@ func userinfoError(err error) error {
 	return errors.New("userinfo: endpoint returned an unusable response")
 }
 
+// verifyError reduces a failed ID token verification to a fixed category.
+// go-oidc's error text can carry the JWKS endpoint's response body or
+// token-supplied values, so only the expiry of an expired token is kept.
+func verifyError(err error) error {
+	var expired *oidc.TokenExpiredError
+	if errors.As(err, &expired) {
+		return fmt.Errorf("verify ID token: token expired at %s", expired.Expiry.UTC().Format(time.RFC3339))
+	}
+	return errors.New("verify ID token: token or provider key set rejected")
+}
+
 // member reports whether groups contains an allowed group.
 func (a *Auth) member(groups []string) bool {
 	return slices.ContainsFunc(groups, func(g string) bool { return slices.Contains(a.allowed, g) })