🧪 test(auth): OIDC handler unit tests (ADR-0028 Phase B.4 follow-up)

This adds the missing unit tests for OIDC HTTP handlers that were deferred
from PR #75. Tests cover unknown provider rejection, missing/unknown state
rejection, and PKCE redirect validation.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

pkg/user/api/oidc_handler_test.go

@@ -0,0 +1,134 @@
+package api
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"dance-lessons-coach/pkg/auth"
+	"dance-lessons-coach/pkg/user"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/stretchr/testify/assert"
+)
+
+// fakeUserSvc is reused from magic_link_handler_test.go
+// It's in the same package (api) so we can use it directly.
+
+// fakeUserRepo is reused from magic_link_handler_test.go
+// It's in the same package (api) so we can use it directly.
+
+// setupMockOIDCProvider creates a mock OIDC provider server for testing.
+// Uses the Q-062 mitigation pattern with var server *httptest.Server.
+func setupMockOIDCProvider(t *testing.T) *httptest.Server {
+	t.Helper()
+	var server *httptest.Server
+	server = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/.well-known/openid-configuration" {
+			w.Header().Set("Content-Type", "application/json")
+			fmt.Fprintf(w, `{"issuer":"%s","authorization_endpoint":"%s/auth","token_endpoint":"%s/token","jwks_uri":"%s/jwks"}`,
+				server.URL, server.URL, server.URL, server.URL)
+			return
+		}
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	return server
+}
+
+// mountOIDCHandler mounts the OIDCHandler on a new router and returns it.
+func mountOIDCHandler(t *testing.T, handler *OIDCHandler) *chi.Mux {
+	t.Helper()
+	r := chi.NewRouter()
+	handler.RegisterRoutes(r)
+	return r
+}
+
+// newTestOIDCHandler creates an OIDCHandler with the given clients.
+func newTestOIDCHandler(clients map[string]*auth.OIDCClient) *OIDCHandler {
+	return NewOIDCHandler(
+		clients,
+		newFakeUserSvc(),
+		&fakeUserRepo{svc: newFakeUserSvc()},
+		"http://localhost:8080",
+	)
+}
+
+// TestOIDCHandler_Start_RejectsUnknownProvider tests that starting with an unknown provider returns 404.
+func TestOIDCHandler_Start_RejectsUnknownProvider(t *testing.T) {
+	handler := newTestOIDCHandler(map[string]*auth.OIDCClient{})
+	router := mountOIDCHandler(t, handler)
+
+	req := httptest.NewRequest(http.MethodGet, "/oidc/unknown/start", nil)
+	rr := httptest.NewRecorder()
+	router.ServeHTTP(rr, req)
+
+	assert.Equal(t, http.StatusNotFound, rr.Code)
+}
+
+// TestOIDCHandler_Callback_RejectsMissingState tests that callback without state returns 401.
+func TestOIDCHandler_Callback_RejectsMissingState(t *testing.T) {
+	client := auth.NewOIDCClient("http://mock-provider", "test-id", "test-secret")
+	handler := newTestOIDCHandler(map[string]*auth.OIDCClient{"test": client})
+	router := mountOIDCHandler(t, handler)
+
+	req := httptest.NewRequest(http.MethodGet, "/oidc/test/callback", nil)
+	rr := httptest.NewRecorder()
+	router.ServeHTTP(rr, req)
+
+	assert.Equal(t, http.StatusUnauthorized, rr.Code)
+}
+
+// TestOIDCHandler_Callback_RejectsUnknownState tests that callback with unknown state returns 401.
+func TestOIDCHandler_Callback_RejectsUnknownState(t *testing.T) {
+	client := auth.NewOIDCClient("http://mock-provider", "test-id", "test-secret")
+	handler := newTestOIDCHandler(map[string]*auth.OIDCClient{"test": client})
+	router := mountOIDCHandler(t, handler)
+
+	req := httptest.NewRequest(http.MethodGet, "/oidc/test/callback?state=unknown&code=any", nil)
+	rr := httptest.NewRecorder()
+	router.ServeHTTP(rr, req)
+
+	assert.Equal(t, http.StatusUnauthorized, rr.Code)
+}
+
+// TestOIDCHandler_Start_RedirectsWithPKCE tests that starting with a valid provider redirects with PKCE.
+func TestOIDCHandler_Start_RedirectsWithPKCE(t *testing.T) {
+	// Setup mock OIDC provider
+	mockServer := setupMockOIDCProvider(t)
+	defer mockServer.Close()
+
+	// Create OIDC client pointing to mock server
+	client := auth.NewOIDCClient(mockServer.URL, "test-id", "test-secret")
+	// Set a custom HTTP client that can reach the mock server
+	client.SetHTTPClient(mockServer.Client())
+
+	handler := newTestOIDCHandler(map[string]*auth.OIDCClient{"test": client})
+	router := mountOIDCHandler(t, handler)
+
+	req := httptest.NewRequest(http.MethodGet, "/oidc/test/start", nil)
+	rr := httptest.NewRecorder()
+	router.ServeHTTP(rr, req)
+
+	// Assert 302 redirect
+	assert.Equal(t, http.StatusFound, rr.Code)
+
+	// Get Location header
+	location := rr.Header().Get("Location")
+	assert.NotEmpty(t, location)
+
+	// Location should start with the mock auth endpoint
+	expectedAuthEndpoint := mockServer.URL + "/auth"
+	assert.Contains(t, location, expectedAuthEndpoint)
+
+	// Location should contain code_challenge and state
+	assert.Contains(t, location, "code_challenge=")
+	assert.Contains(t, location, "state=")
+	assert.Contains(t, location, "response_type=code")
+	assert.Contains(t, location, "client_id=test-id")
+	assert.Contains(t, location, "code_challenge_method=S256")
+}
+
+// Ensure the interfaces are satisfied at compile time
+var _ user.UserService = (*fakeUserSvc)(nil)
+var _ user.UserRepository = (*fakeUserRepo)(nil)