feat: Bulk actions for contacts (#12763)

Introduces APIs and UI for bulk actions in contacts table. The initial
action available will be assign labels

Fixes: #8536 #12253 

## Screens

<img width="1350" height="747" alt="Screenshot 2025-10-29 at 4 05 08 PM"
src="https://github.com/user-attachments/assets/0792dff5-0371-4b2e-bdfb-cd32db773402"
/>
<img width="1345" height="717" alt="Screenshot 2025-10-29 at 4 05 19 PM"
src="https://github.com/user-attachments/assets/ae510404-c6de-4c15-a720-f6d10cdac25b"
/>

---------

Co-authored-by: Muhsin <muhsinkeramam@gmail.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

spec/services/contacts/bulk_assign_labels_service_spec.rb

@@ -0,0 +1,48 @@
+require 'rails_helper'
+
+RSpec.describe Contacts::BulkAssignLabelsService do
+  subject(:service) do
+    described_class.new(
+      account: account,
+      contact_ids: [contact_one.id, contact_two.id, other_contact.id],
+      labels: labels
+    )
+  end
+
+  let(:account) { create(:account) }
+  let!(:contact_one) { create(:contact, account: account) }
+  let!(:contact_two) { create(:contact, account: account) }
+  let!(:other_contact) { create(:contact) }
+  let(:labels) { %w[vip support] }
+
+  it 'assigns labels to the contacts that belong to the account' do
+    service.perform
+
+    expect(contact_one.reload.label_list).to include(*labels)
+    expect(contact_two.reload.label_list).to include(*labels)
+  end
+
+  it 'does not assign labels to contacts outside the account' do
+    service.perform
+
+    expect(other_contact.reload.label_list).to be_empty
+  end
+
+  it 'returns ids of contacts that were updated' do
+    result = service.perform
+
+    expect(result[:success]).to be(true)
+    expect(result[:updated_contact_ids]).to contain_exactly(contact_one.id, contact_two.id)
+  end
+
+  it 'returns success with no updates when labels are blank' do
+    result = described_class.new(
+      account: account,
+      contact_ids: [contact_one.id],
+      labels: []
+    ).perform
+
+    expect(result).to eq(success: true, updated_contact_ids: [])
+    expect(contact_one.reload.label_list).to be_empty
+  end
+end