fix(connector): relax Slack incoming webhook requirement

ParseSlackTokenResponse treated the incoming_webhook field as
mandatory, which blocked any Slack OAuth2 flow that did not request
the incoming-webhook scope. Access review Slack connects only ask
for users:read and users:read.email and would fail at token parsing.

Treat incoming_webhook as optional: populate SlackSettings when it
is present, leave them empty otherwise. The existing compliance-page
webhook URL is preserved through Reconnect in a later commit.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/slack_test.go

@@ -0,0 +1,67 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package connector
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.probo.inc/probo/pkg/gid"
+)
+
+func TestParseSlackTokenResponse(t *testing.T) {
+	t.Parallel()
+
+	orgID := gid.New(gid.NewTenantID(), 0)
+	base := OAuth2Connection{
+		AccessToken: "xoxb-test",
+		TokenType:   "bot",
+		ExpiresAt:   time.Now().Add(time.Hour),
+		Scope:       "chat:write channels:join incoming-webhook",
+	}
+
+	t.Run("with incoming webhook", func(t *testing.T) {
+		t.Parallel()
+		body := []byte(`{"ok":true,"incoming_webhook":{"url":"https://hooks.slack.com/services/T/B/X","channel":"#general","channel_id":"C123"}}`)
+
+		conn, returnedOrgID, err := ParseSlackTokenResponse(body, base, orgID)
+		require.NoError(t, err)
+		require.NotNil(t, conn)
+		require.NotNil(t, returnedOrgID)
+
+		assert.Equal(t, orgID, *returnedOrgID)
+		assert.Equal(t, "https://hooks.slack.com/services/T/B/X", conn.Settings.WebhookURL)
+		assert.Equal(t, "#general", conn.Settings.Channel)
+		assert.Equal(t, "C123", conn.Settings.ChannelID)
+		assert.Equal(t, "xoxb-test", conn.AccessToken)
+	})
+
+	t.Run("without incoming webhook", func(t *testing.T) {
+		t.Parallel()
+		body := []byte(`{"ok":true}`)
+
+		conn, returnedOrgID, err := ParseSlackTokenResponse(body, base, orgID)
+		require.NoError(t, err)
+		require.NotNil(t, conn)
+		require.NotNil(t, returnedOrgID)
+
+		assert.Empty(t, conn.Settings.WebhookURL)
+		assert.Empty(t, conn.Settings.Channel)
+		assert.Empty(t, conn.Settings.ChannelID)
+		assert.Equal(t, "xoxb-test", conn.AccessToken)
+	})
+}