MM-63912 - enhance validation for channel banner colors (#30981)

* MM-63912 - enhance validation for channel banner colors

* fix tests

* fix unit tests

server/public/model/channel_test.go

@@ -87,6 +87,68 @@ func TestChannelIsValid(t *testing.T) {
 	require.NotNil(t, o.IsValid())
 }
 
+func TestChannelBannerBackgroundColorValidation(t *testing.T) {
+	o := Channel{
+		Id:       NewId(),
+		CreateAt: GetMillis(),
+		UpdateAt: GetMillis(),
+		Name:     "valid-name",
+		Type:     ChannelTypeOpen,
+		Header:   "valid-header",
+		Purpose:  "valid-purpose",
+		BannerInfo: &ChannelBannerInfo{
+			Enabled: NewPointer(true),
+			Text:    NewPointer("Banner Text"),
+		},
+	}
+
+	// Test with nil background color
+	o.BannerInfo.BackgroundColor = nil
+	require.NotNil(t, o.IsValid())
+	require.Equal(t, "model.channel.is_valid.banner_info.background_color.empty.app_error", o.IsValid().Id)
+
+	// Test with empty background color
+	o.BannerInfo.BackgroundColor = NewPointer("")
+	require.NotNil(t, o.IsValid())
+	require.Equal(t, "model.channel.is_valid.banner_info.background_color.empty.app_error", o.IsValid().Id)
+
+	// Test with invalid background color (no # prefix)
+	o.BannerInfo.BackgroundColor = NewPointer("FF0000")
+	require.NotNil(t, o.IsValid())
+	require.Equal(t, "model.channel.is_valid.banner_info.background_color.invalid.app_error", o.IsValid().Id)
+
+	// Test with invalid background color (invalid characters)
+	o.BannerInfo.BackgroundColor = NewPointer("#GGGGGG")
+	require.NotNil(t, o.IsValid())
+	require.Equal(t, "model.channel.is_valid.banner_info.background_color.invalid.app_error", o.IsValid().Id)
+
+	// Test with invalid background color (wrong length)
+	o.BannerInfo.BackgroundColor = NewPointer("#FF00")
+	require.NotNil(t, o.IsValid())
+	require.Equal(t, "model.channel.is_valid.banner_info.background_color.invalid.app_error", o.IsValid().Id)
+
+	// Test with invalid background color (wrong length)
+	o.BannerInfo.BackgroundColor = NewPointer("#FF00000")
+	require.NotNil(t, o.IsValid())
+	require.Equal(t, "model.channel.is_valid.banner_info.background_color.invalid.app_error", o.IsValid().Id)
+
+	// Test with valid 6-digit hex color
+	o.BannerInfo.BackgroundColor = NewPointer("#FF0000")
+	require.Nil(t, o.IsValid())
+
+	// Test with valid 6-digit hex color (lowercase)
+	o.BannerInfo.BackgroundColor = NewPointer("#ff0000")
+	require.Nil(t, o.IsValid())
+
+	// Test with valid 3-digit hex color
+	o.BannerInfo.BackgroundColor = NewPointer("#F00")
+	require.Nil(t, o.IsValid())
+
+	// Test with valid 3-digit hex color (lowercase)
+	o.BannerInfo.BackgroundColor = NewPointer("#f00")
+	require.Nil(t, o.IsValid())
+}
+
 func TestChannelPreSave(t *testing.T) {
 	o := Channel{Name: "test"}
 	o.PreSave()