Make auth cookie SameSite configurable

Add same-site to auth cookie config with lax as the default,
PROBOD_AUTH_COOKIE_SAMESITE bootstrap mapping, and validation
that rejects none unless Secure is enabled.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/probodconfig/auth_config_test.go

@@ -0,0 +1,121 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package probodconfig_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"go.probo.inc/probo/pkg/probodconfig"
+)
+
+func TestParseCookieSameSite(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		input   string
+		want    probodconfig.CookieSameSite
+		wantErr bool
+	}{
+		{name: "empty", input: "", want: probodconfig.CookieSameSiteLax},
+		{name: "lax", input: "lax", want: probodconfig.CookieSameSiteLax},
+		{name: "Lax", input: "Lax", want: probodconfig.CookieSameSiteLax},
+		{name: "strict", input: "strict", want: probodconfig.CookieSameSiteStrict},
+		{name: "none", input: "none", want: probodconfig.CookieSameSiteNone},
+		{name: "invalid", input: "cross-site", wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := probodconfig.ParseCookieSameSite(tt.input)
+			if tt.wantErr {
+				require.Error(t, err)
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}
+
+func TestCookieConfig_Validate(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		cookie  probodconfig.CookieConfig
+		wantErr bool
+	}{
+		{
+			name: "lax without secure",
+			cookie: probodconfig.CookieConfig{
+				SameSite: probodconfig.CookieSameSiteLax,
+				Secure:   false,
+			},
+		},
+		{
+			name: "none requires secure",
+			cookie: probodconfig.CookieConfig{
+				SameSite: probodconfig.CookieSameSiteNone,
+				Secure:   false,
+			},
+			wantErr: true,
+		},
+		{
+			name: "none with secure",
+			cookie: probodconfig.CookieConfig{
+				SameSite: probodconfig.CookieSameSiteNone,
+				Secure:   true,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := tt.cookie.Validate()
+			if tt.wantErr {
+				require.Error(t, err)
+				return
+			}
+
+			require.NoError(t, err)
+		})
+	}
+}
+
+func TestCookieConfig_HTTPSameSite(t *testing.T) {
+	t.Parallel()
+
+	cookie := probodconfig.CookieConfig{SameSite: probodconfig.CookieSameSiteStrict}
+
+	got, err := cookie.HTTPSameSite()
+	require.NoError(t, err)
+	assert.Equal(t, http.SameSiteStrictMode, got)
+}