refactor: improve token file reading with utils

Replace direct vim file reading and JSON decoding functions with utility
functions for better error handling and code consistency. This change makes
the token retrieval more robust by safely handling file reading and
JSON parsing failures.

Author: deathbeam

lua/CopilotChat/config/providers.lua

@@ -86,12 +86,15 @@ local function get_github_token()
   }
 
   for _, file_path in ipairs(file_paths) do
-    if vim.fn.filereadable(file_path) == 1 then
-      local userdata = vim.fn.json_decode(vim.fn.readfile(file_path))
-      for key, value in pairs(userdata) do
-        if string.find(key, 'github.com') then
-          cached_github_token = value.oauth_token
-          return value.oauth_token
+    local file_data = utils.read_file(file_path)
+    if file_data then
+      local parsed_data = utils.json_decode(file_data)
+      if parsed_data then
+        for key, value in pairs(parsed_data) do
+          if string.find(key, 'github.com') then
+            cached_github_token = value.oauth_token
+            return value.oauth_token
+          end
         end
       end
     end