fix(lsp): ignore inline completions after leaving insert mode (#36476)

**Problem:** When quickly entering and leaving insert mode, sometimes
inline completion requests are returned and handled in normal mode. This
causes an extmark to be set, which will not get cleared until the next
time entering & leaving insert mode.

**Solution:** Return early in the inline completion handler if we have
left insert mode.

runtime/lua/vim/lsp/inline_completion.lua

@@ -121,7 +121,7 @@ function Completor:handler(err, result, ctx)
     log.error('inlinecompletion', err)
     return
   end
-  if not result then
+  if not result or not vim.startswith(api.nvim_get_mode().mode, 'i') then
     return
   end