fix: use :startinsert instead of feedkeys to switch to terminal mode

If user has their own `startinsert`, e.g. in the following case,
`startinsert` is trigger by `:term`, but later fzf term is focused.
```
nvim \
  +"au TermOpen * startinsert" \
  +"term" \
  +"lua vim.defer_fn(function() vim.cmd [[FzfLua files]] end, 100)"
```
Technically user should not `startinsert` on an incorrect window, but
currently there's no way to avoid it since builtin `startinsert` seems
defered but `nvim_get_mode` is fast (return actual mode immediately).

This PR periodly retry `startinsert` until `nt` mode on fzf term buffer.

TODO: test

Author: phanen

lua/fzf-lua/fzf.lua

@@ -333,14 +333,8 @@ function M.raw_fzf(contents, fzf_cli_args, opts)
   -- fzf-tmux spawns outside neovim, don't set filetype/insert mode
   if not opts.is_fzf_tmux then
     vim.bo.filetype = "fzf"
-
     -- See note in "ModeChanged" above
-    if vim.api.nvim_get_mode().mode == "t" then
-      -- Called from another fzf-win most likely
-      utils.feed_keys_termcodes("i")
-    else
-      vim.cmd [[startinsert]]
-    end
+    utils.ensure_startinsert(5, 10)
   end
 
   return coroutine.yield()

lua/fzf-lua/utils.lua

@@ -1298,6 +1298,26 @@ function M.setmetatable__gc(t, mt)
   return setmetatable(t, mt)
 end
 
+---@param retry integer?
+---@param timeout integer?
+function M.ensure_startinsert(retry, timeout)
+  local bufnr = vim.api.nvim_get_current_buf()
+  local function ensure_startinsert(_retry, _timeout)
+    -- abort for some reason we switch to another buffer
+    if vim.api.nvim_get_current_buf() ~= bufnr then return end
+    if vim.api.nvim_get_mode().mode ~= "t" then
+      vim.cmd [[startinsert]]
+      return
+    end
+    -- Called from another fzf-win most likely
+    -- don't startinsert until we enter new terminal mode
+    return vim.defer_fn(function()
+      return ensure_startinsert(_retry - 1, math.min(100, _timeout * 2))
+    end, _timeout)
+  end
+  return ensure_startinsert(retry or 5, timeout or 10)
+end
+
 --- Checks if treesitter parser for language is installed
 ---@param lang string
 function M.has_ts_parser(lang)