Do not fail import of merge request when unsupported diff note occurs

Some validations doesn't make sense during a project import, since the
objects are built in a custom way.

Related to: #285107

app/models/diff_note.rb

@@ -19,7 +19,7 @@ def self.noteable_types
   # EE might have added a type when the module was prepended
   validates :noteable_type, inclusion: { in: -> (_note) { noteable_types } }
   validate :positions_complete
-  validate :verify_supported
+  validate :verify_supported, unless: :importing?
 
   before_validation :set_line_code, if: :on_text?, unless: :importing?
   after_save :keep_around_commits, unless: :importing?

app/models/system_note_metadata.rb

 # frozen_string_literal: true
 
 class SystemNoteMetadata < ApplicationRecord
+  include Importable
+
   # These notes's action text might contain a reference that is external.
   # We should always force a deep validation upon references that are found
   # in this note type.
@@ -23,7 +25,7 @@ class SystemNoteMetadata < ApplicationRecord
     status alert_issue_added relate unrelate new_alert_added severity
   ].freeze
 
-  validates :note, presence: true
+  validates :note, presence: true, unless: :importing?
   validates :action, inclusion: { in: :icon_types }, allow_nil: true
 
   belongs_to :note

changelogs/unreleased/kassio-avoid-invalid-notes-when-importing-a-project.yml

+---
+title: Avoid invalid notes on Project Import
+merge_request: 48189
+author:
+type: fixed

spec/models/diff_note_spec.rb

@@ -46,6 +46,18 @@
 
       expect(note.errors[:noteable]).to include("doesn't support new-style diff notes")
     end
+
+    context 'when importing' do
+      it "does not check if it's supported" do
+        note = build(:diff_note_on_merge_request, project: project, noteable: nil)
+        note.importing = true
+        note.valid?
+
+        expect(note.errors.full_messages).not_to include(
+          "Noteable doesn't support new-style diff notes"
+        )
+      end
+    end
   end
 
   describe "#position=" do

spec/models/system_note_metadata_spec.rb

@@ -13,7 +13,7 @@
 
     context 'when action type is invalid' do
       subject do
-        build(:system_note_metadata, note: build(:note), action: 'invalid_type' )
+        build(:system_note_metadata, note: build(:note), action: 'invalid_type')
       end
 
       it { is_expected.to be_invalid }
@@ -21,7 +21,15 @@
 
     context 'when action type is valid' do
       subject do
-        build(:system_note_metadata, note: build(:note), action: 'merge' )
+        build(:system_note_metadata, note: build(:note), action: 'merge')
+      end
+
+      it { is_expected.to be_valid }
+    end
+
+    context 'when importing' do
+      subject do
+        build(:system_note_metadata, note: nil, action: 'merge', importing: true)
       end
 
       it { is_expected.to be_valid }