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/action_service_spec.rb

@@ -112,4 +112,15 @@ describe ActionService do
       end
     end
   end
+
+  describe '#remove_assigned_agent' do
+    let(:conversation) { create(:conversation, :with_assignee, account: account) }
+    let(:action_service) { described_class.new(conversation) }
+
+    it 'unassigns the conversation' do
+      expect(conversation.reload.assignee).to be_present
+      action_service.remove_assigned_agent(nil)
+      expect(conversation.reload.assignee).to be_nil
+    end
+  end
 end