fix(vim.text): remove assert from vim.text.hexdecode

Instead, return nil plus an error message if the input is invalid.

runtime/doc/lua.txt

@@ -4385,8 +4385,9 @@ vim.text.hexdecode({enc})                               *vim.text.hexdecode()*
     Parameters: ~
       • {enc}  (`string`) String to decode
 
-    Return: ~
-        (`string`) Decoded string
+    Return (multiple): ~
+        (`string?`) Decoded string
+        (`string?`) Error message, if any
 
 vim.text.hexencode({str})                               *vim.text.hexencode()*
     Hex encode a string.

runtime/lua/vim/text.lua

@@ -18,15 +18,19 @@ end
 --- Hex decode a string.
 ---
 --- @param enc string String to decode
---- @return string : Decoded string
+--- @return string? : Decoded string
+--- @return string? : Error message, if any
 function M.hexdecode(enc)
-  assert(#enc % 2 == 0, 'string must have an even number of hex characters')
+  if #enc % 2 ~= 0 then
+    return nil, 'string must have an even number of hex characters'
+  end
+
   local str = {} ---@type string[]
   for i = 1, #enc, 2 do
     local n = assert(tonumber(enc:sub(i, i + 1), 16))
     str[#str + 1] = string.char(n)
   end
-  return table.concat(str)
+  return table.concat(str), nil
 end
 
 return M