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.

app/services/action_service.rb

@@ -60,8 +60,7 @@ class ActionService
   end
 
   def assign_team(team_ids = [])
-    # FIXME: The explicit checks for zero or nil (string) is bad. Move
-    # this to a separate unassign action.
+    # Keep nil/0 handling for existing automation and macro payloads.
     should_unassign = team_ids.blank? || %w[nil 0].include?(team_ids[0].to_s)
     return @conversation.update!(team_id: nil) if should_unassign
 
@@ -72,6 +71,10 @@ class ActionService
     @conversation.update!(team_id: team_ids[0])
   end
 
+  def remove_assigned_agent(_params)
+    @conversation.update!(assignee_id: nil)
+  end
+
   def remove_assigned_team(_params)
     @conversation.update!(team_id: nil)
   end