feat(system): improve system command context error handling

Display error messages when system commands fail, showing the stderr
output when available, or a generic failure message with the exit code
when stderr is empty. Additionally, differentiate between successful and
failed commands by using distinct filenames for better context in the
Copilot Chat UI.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

lua/CopilotChat/config/contexts.lua

@@ -324,14 +324,25 @@ return {
       end
 
       local out = utils.system({ shell, shell_flag, input })
-      if not out or out.stdout == '' then
+      if not out then
         return {}
       end
 
+      local out_type = 'command_output'
+      local out_text = out.stdout
+      if out.code ~= 0 then
+        out_type = 'command_error'
+        if out.stderr and out.stderr ~= '' then
+          out_text = out.stderr
+        elseif not out_text or out_text == '' then
+          out_text = 'Command failed with exit code ' .. out.code
+        end
+      end
+
       return {
         {
-          content = out.stdout,
-          filename = 'command_output_' .. input:gsub('[^%w]', '_'):sub(1, 20),
+          content = out_text,
+          filename = out_type .. '_' .. input:gsub('[^%w]', '_'):sub(1, 20),
           filetype = 'text',
         },
       }