refactor(ui): improve chat and overlay function signatures (#1400)

- Remove unnecessary visibility check in Chat:remove_message
- Add @Protected annotation to Chat:parse for clarity
- Make Overlay:show_help pos parameter optional for flexibility

These changes enhance code readability and maintainability by refining
function signatures and documentation.

Author: deathbeam

lua/CopilotChat/ui/chat.lua

@@ -516,11 +516,8 @@ end
 ---@param role string? If specified, only considers sections of the given role
 ---@param cursor boolean? If true, removes the message closest to the cursor position
 function Chat:remove_message(role, cursor)
-  if not self:visible() then
-    return
-  end
-
   self:parse()
+
   local message = self:get_message(role, cursor)
   if not message then
     return
@@ -615,6 +612,8 @@ function Chat:validate()
   end
 end
 
+--- Parse the chat window buffer into structured messages.
+---@protected
 function Chat:parse()
   self:validate()
 

lua/CopilotChat/ui/overlay.lua

@@ -130,7 +130,7 @@ end
 
 --- Show help message in the overlay
 ---@param msg string?
----@param pos number
+---@param pos number?
 ---@protected
 function Overlay:show_help(msg, pos)
   if not msg or msg == '' then