fix: add explicit remove assignment actions to macros and automations (#12172)

This updates macros and automations so agents can explicitly remove
assigned agents or teams, while keeping the existing `Assign -> None`
flow working for backward compatibility.

Fixes: #7551
Closes: #7551

## Why
The original macro change exposed unassignment only through `Assign ->
None`, which made macros behave differently from automations and left
the explicit remove actions inconsistent across the product. This keeps
the lower-risk compatibility path and adds the explicit remove actions
requested in review.

## What this change does
- Adds `Remove Assigned Agent` and `Remove Assigned Team` as explicit
actions in macros.
- Adds the same explicit remove actions in automations.
- Keeps `Assign Agent -> None` and `Assign Team -> None` working for
existing behavior and stored payloads.
- Preserves backward compatibility for existing macro and automation
execution payloads.
- Downmerges the latest `develop` and resolves the conflicts while
keeping both the new remove actions and current `develop` behavior.

## Validation
- Verified both remove actions are available and selectable in the macro
editor.
- Verified both remove actions are available and selectable in the
automation builder.
- Applied a disposable macro with `Remove Assigned Agent` and `Remove
Assigned Team` on a real conversation and confirmed both fields were
cleared.
- Applied a disposable macro with `Assign Agent -> None` and `Assign
Team -> None` on a real conversation and confirmed both fields were
still cleared.

spec/services/macros/execution_service_spec.rb

@@ -49,6 +49,18 @@ RSpec.describe Macros::ExecutionService, type: :service do
     end
   end
 
+  describe '#assign_team' do
+    let(:team) { create(:team, account: account, allow_auto_assign: false) }
+
+    context 'when team_id is nil' do
+      it 'unassigns the team from the conversation' do
+        conversation.update!(team_id: team.id)
+        service.send(:assign_team, ['nil'])
+        expect(conversation.reload.team).to be_nil
+      end
+    end
+  end
+
   describe '#assign_agent' do
     context 'when agent_ids contains self' do
       it 'updates the conversation assignee to the current user' do
@@ -69,6 +81,14 @@ RSpec.describe Macros::ExecutionService, type: :service do
         expect(conversation.reload.assignee).to eq(other_user)
       end
     end
+
+    context 'when agent_ids contains nil' do
+      it 'unassigns the conversation' do
+        conversation.update!(assignee: user)
+        service.send(:assign_agent, ['nil'])
+        expect(conversation.reload.assignee).to be_nil
+      end
+    end
   end
 
   describe '#add_private_note' do