fix(ui): handle missing filename in chat block header (#1406)

Previously, chat blocks without a filename would display 'block' as the
filename. This change updates the logic to use nil for missing filenames
and ensures the UI displays 'block' only when filename is not present.
This improves clarity and consistency in chat block rendering.

Author: deathbeam

lua/CopilotChat/ui/chat.lua

@@ -48,7 +48,7 @@ local function match_block_header(header)
     if path then
       return type, path, tonumber(start_line) or 1, tonumber(end_line) or tonumber(start_line) or 1
     elseif type then
-      return type, 'block'
+      return type, nil
     end
   end
 end
@@ -801,7 +801,7 @@ function Chat:render()
         local header = block.header
         local filetype = header.filetype
         local filename = header.filename
-        local text = string.format('[%s] %s', filetype, filename)
+        local text = string.format('[%s] %s', filetype, filename or 'block')
         if header.start_line and header.end_line then
           text = text .. string.format(' lines %d-%d', header.start_line, header.end_line)
         end