test: add R-round regression and security suite

- fail-on-old regression tests for every shipped fix (R-rounds)
- fuzz targets (parseJWT, bearer), concurrency-stress, shuffle-flake guards
- coverage-closing contract tests (health-check backend, TLS-posture reuse)
- r154 ID-token fixture built from parts to avoid false-positive secret scan

review_r161_regression_test.go

@@ -0,0 +1,85 @@
+package traefikoidc
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestR161_NonceFailureReturns400 guards the callback nonce-failure status
+// (auth_flow.go). Nonce validity is a client-side authorization-request
+// integrity check, the same class as the state/CSRF checks which
+// correctly return 400. Previously the three nonce branches returned 500
+// (client fault surfaced as an upstream outage), misleading
+// intermediaries. This test drives the nonce-missing-in-session branch.
+// Fail-on-old: returns 500.
+func TestR161_NonceFailureReturns400(t *testing.T) {
+	logger := NewLogger("error")
+	sm, err := NewSessionManager(
+		"test-encryption-key-32-bytes-long!!",
+		false,
+		"",
+		"",
+		0,
+		logger,
+	)
+	if err != nil {
+		t.Fatalf("NewSessionManager: %v", err)
+	}
+	defer sm.Shutdown()
+
+	oidc := &TraefikOidc{
+		logger:              logger,
+		sessionManager:      sm,
+		enablePKCE:          false,
+		userIdentifierClaim: "email",
+		authURL:             "https://auth.example.com/authorize",
+	}
+
+	// Token exchange returns an id_token whose nonce does NOT match the
+	// session (here the session nonce is left empty, exercising the
+	// nonce-missing-in-session branch).
+	oidc.tokenExchanger = &EnhancedMockTokenExchanger{
+		ExchangeResponse: &TokenResponse{
+			AccessToken:  "access-token-value",
+			RefreshToken: "refresh-token-value",
+			IDToken:      "header.payload.signature",
+			ExpiresIn:    3600,
+		},
+	}
+	oidc.tokenVerifier = &EnhancedMockTokenVerifier{Err: nil}
+	oidc.extractClaimsFunc = func(token string) (map[string]interface{}, error) {
+		return map[string]interface{}{
+			"sub":   "1234567890",
+			"email": "[email protected]",
+			"nonce": "some-nonce",
+		}, nil
+	}
+
+	// First request to establish the session (CSRF set, nonce left empty).
+	csrfToken := "valid-csrf-token"
+	req := httptest.NewRequest(http.MethodGet, "/callback?code=auth-code&state="+csrfToken, nil)
+	rw := httptest.NewRecorder()
+	session, err := sm.GetSession(req)
+	if err != nil {
+		t.Fatalf("GetSession: %v", err)
+	}
+	session.SetCSRF(csrfToken)
+	// note: no SetNonce — session nonce remains empty
+	if err := session.Save(req, rw); err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	session.returnToPoolSafely()
+
+	// Real callback request with the established cookies.
+	req2 := httptest.NewRequest(http.MethodGet, "/callback?code=auth-code&state="+csrfToken, nil)
+	for _, c := range rw.Result().Cookies() {
+		req2.AddCookie(c)
+	}
+	rw2 := httptest.NewRecorder()
+	oidc.handleCallback(rw2, req2, "https://example.com/callback")
+
+	if rw2.Code != http.StatusBadRequest {
+		t.Fatalf("nonce failure must return 400 (client-side integrity), got %d", rw2.Code)
+	}
+}