fix(vim.text): handle very long strings (#30075)

Lua's string.byte has a maximum (undocumented) allowable length, so
vim.text.hencode fails on large strings with the error "string slice too
long".

Instead of converting the string to an array of bytes up front, convert
each character to a byte one at a time.

runtime/lua/vim/text.lua

@@ -7,10 +7,9 @@ local M = {}
 --- @param str string String to encode
 --- @return string : Hex encoded string
 function M.hexencode(str)
-  local bytes = { str:byte(1, #str) }
   local enc = {} ---@type string[]
-  for i = 1, #bytes do
-    enc[i] = string.format('%02X', bytes[i])
+  for i = 1, #str do
+    enc[i] = string.format('%02X', str:byte(i, i + 1))
   end
   return table.concat(enc)
 end

test/functional/lua/text_spec.lua

@@ -20,5 +20,11 @@ describe('vim.text', function()
         eq(input, vim.text.hexdecode(output))
       end
     end)
+
+    it('works with very large strings', function()
+      local input, output = string.rep('😂', 2 ^ 16), string.rep('F09F9882', 2 ^ 16)
+      eq(output, vim.text.hexencode(input))
+      eq(input, vim.text.hexdecode(output))
+    end)
   end)
 end)