Added note_tag factory and NoteTag model test-case

Added registering new factory bot for note_tag and added new unit tests
to NoteTagTest for checking if key length is valid, value length is
valid, key length is invalid, value length is invalid, orphaned tag is
invalid and note_tags are unique.

test/factories/note_tags.rb

@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :note_tag do
+    sequence(:k) { |n| "Key #{n}" }
+    sequence(:v) { |n| "Value #{n}" }
+
+    note
+  end
+end

test/models/note_tag_test.rb

@@ -1,7 +1,49 @@
 require "test_helper"
 
 class NoteTagTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  def test_length_key_valid
+    tag = create(:note_tag)
+    [0, 255].each do |i|
+      tag.k = "k" * i
+      assert_predicate tag, :valid?
+    end
+  end
+
+  def test_length_value_valid
+    tag = create(:note_tag)
+    [0, 255].each do |i|
+      tag.v = "v" * i
+      assert_predicate tag, :valid?
+    end
+  end
+
+  def test_length_key_invalid
+    tag = create(:note_tag)
+    tag.k = "k" * 256
+    assert_not_predicate tag, :valid?, "Key should be too long"
+    assert_predicate tag.errors[:k], :any?
+  end
+
+  def test_length_value_invalid
+    tag = create(:note_tag)
+    tag.v = "v" * 256
+    assert_not_predicate tag, :valid?, "Value should be too long"
+    assert_predicate tag.errors[:v], :any?
+  end
+
+  def test_orphaned_tag_invalid
+    tag = create(:note_tag)
+    tag.note = nil
+    assert_not_predicate tag, :valid?, "Orphaned tag should be invalid"
+    assert_predicate tag.errors[:note], :any?
+  end
+
+  def test_uniqueness
+    existing = create(:note_tag)
+    tag = build(:note_tag, :note => existing.note, :k => existing.k, :v => existing.v)
+    assert_predicate tag, :new_record?
+    assert_not_predicate tag, :valid?
+    assert_raise(ActiveRecord::RecordInvalid) { tag.save! }
+    assert_predicate tag, :new_record?
+  end
 end