contact_form.rb

class ContactForm
  include ActiveModel::Model
  include ActiveModel::Attributes
  #include ActiveModel::Callbacks

  attr_accessor :name, :email, :subject, :body, :agreement_privacy_policy
  define_model_callbacks :save

  validates :name, presence: true
  validates :email, presence: true
  validates :subject, presence: true
  validates :body, presence: true
  validates :agreement_privacy_policy, presence: true

  validates_acceptance_of :agreement_privacy_policy, allow_nil: false
  def save
    run_callbacks :save do
      return false if invalid?
      @name = self.name
      @email = self.email
      @subject = self.subject
      @body = self.body

      bodyAttachments = {"title": "メンターお問いあわせ",
                          "text": "\n\n\n*Email* :\n" + @email +
                          "\n\n*お名前* :\n" + @name +
                          "\n\n*件名* :\n" + @subject +
                          "\n\n*本文* :\n" + @body
                          };

      massage_body = "<!channel> お問い合わせフォームより新しい通知が来ています！ \n"

      notifier = slack_notifer_client "#各種お問い合わせボード"
      notifier.post attachments: [bodyAttachments], text: massage_body

      true
    end
  end
  private
    def slack_notifer_client channel
      hooks_url = ENV['SLACK_INCOMING_WEBHOOKS_URL']
      Slack::Notifier.new hooks_url, channel: channel
    end
end

contact_form_spec.rb

require 'rails_helper'

RSpec.describe ContactForm, type: :model do
  describe "#Save" do

    let(:contact_form) { ContactForm.new(params) }

    let(:params) { { name: name, email: email, subject: subject, body: body } }

    let(:slack_notifer_client_mock) { double('Slack Notifer client') }

    before do
      allow(slack_notifer_client_mock).to receive(:post)
      allow_any_instance_of(ContactForm).to receive(:slack_notifer_client).and_return(slack_notifer_client_mock)
    end

    before :each do
      contact_form.save
    end

    context "Name が空欄の時" do
      let(:name) {  }
      let(:email) { "[email protected]" }
      let(:subject) { "Subject" }
      let(:body) { "ABCDEF BODY" }

      it "エラーを返すこと" do
        Rails.logger.info contact_form
        expect(contact_form.errors[:name]).to be_present
      end
    end

    context "E-mail が空欄の時" do
      let(:name) { "ABC NAME" }
      let(:email) {  }
      let(:subject) { "Subject" }
      let(:body) { "ABCDEF BODY" }

      it "エラーを返すこと" do
        expect(contact_form.errors[:email]).to be_present
      end
    end

    context "Subject が空欄の時" do
      let(:name) { "ABC NAME" }
      let(:email) { "[email protected]" }
      let(:subject) {  }
      let(:body) { "ABCDEF BODY" }

      it "エラーを返すこと" do
        expect(contact_form.errors[:subject]).to be_present
      end
    end

    context "Body が空欄の時" do
      let(:name) { "ABC NAME" }
      let(:email) { "[email protected]" }
      let(:subject) { "Subject" }
      let(:body) {  }

      it "エラーを返すこと" do
        Rails.logger.info contact_form
        contact_form.save
        expect(contact_form.errors[:body]).to be_present
      end
    end

    context "全ての項目が埋まっている時" do
      let(:name) { "ABC NAME" }
      let(:email) { "[email protected]" }
      let(:subject) { "Subject" }
      let(:body) { "ABCDEF BODY" }

      let(:massage_body) { "<!channel> お問い合わせフォームより新しい通知が来ています！ \n" }

      let(:bodyAttachments) {
        {"title": "メンターお問いあわせ",
          "text": "\n\n\n*Email* :\n" + email +
          "\n\n*お名前* :\n" + name +
          "\n\n*件名* :\n" + subject +
          "\n\n*本文* :\n" + body
        }
      }

      it "フォームが正しく送信されること" do
        expect{ contact_form.save }.not_to raise_error
      end

      it "Slack に正しい情報が渡されること" do
        expect(slack_notifer_client_mock).to receive(:post)
      end

      it "それぞれのインスタンス変数に正しい値が代入されていること" do
        expect(contact_form.instance_variable_get :@name).to eq name
        expect(contact_form.instance_variable_get :@email).to eq email
        expect(contact_form.instance_variable_get :@subject).to eq subject
        expect(contact_form.instance_variable_get :@body).to eq body
      end
    end
  end
end

result.sh

bundle exec rspec spec

##### RESULT #####

Failures:

  1) ContactForm#Save 全ての項目が埋まっている時 Slack に正しい情報が渡されること
     Failure/Error: expect(slack_notifer_client_mock).to receive(:post)

       (Double "Slack Notifer client").post(*(any args))
           expected: 1 time with any arguments
           received: 0 times with any arguments
     # ./spec/models/contact_form_spec.rb:90:in `block (4 levels) in <top (required)>'
     
Finished in 1.36 seconds (files took 5.27 seconds to load)
45 examples, 1 failure, 15 pending

Failed examples:

rspec ./spec/models/contact_form_spec.rb:89 # ContactForm#Save 全ての項目が埋まっている時 Slack に正しい情報が渡されること

どこがだめ何だろうとか色々考えるけどわからない...
Mock？ リアルだと正しく動くからな... 🤔 💭

答えは簡単だった。

class ContactForm
  include ActiveModel::Model
 # 略

  attr_accessor :name, :email, :subject, :body, :agreement_privacy_policy
  define_model_callbacks :save

  validates :name, presence: true
  validates :email, presence: true
  validates :subject, presence: true
  validates :body, presence: true
  validates :agreement_privacy_policy, presence: true

  validates_acceptance_of :agreement_privacy_policy, allow_nil: false
  def save
 # 後略

ここで、agreemnt_privacy_policy を、 presence: true かつ、 allow_nil: false って指定してるのにもかかわらず、

require 'rails_helper'

RSpec.describe ContactForm, type: :model do
  describe "#Save" do

    let(:contact_form) { ContactForm.new(params) }

    let(:params) { { name: name, email: email, subject: subject, body: body } }

    let(:slack_notifer_client_mock) { double('Slack Notifer client') }
 # 後略 

Spec 側で、 let(:params) { { name: name, email: email, subject: subject, body: body } } の部分にagreemnt_privacy_policy: true としてなかったので、Model 側の save メソッドで、 return false if invalid? の評価式ですでに false が返されてたっていうだけ。