fix(terminal): avoid mismatched busy_start without busy_stop (#32458)

Problem: `showmode` in `terminal_enter` may cause `vpeekc` to process events,
which may handle pending escape sequences. If `CSI ? 25 l` is handled to hide
the cursor, it may remain hidden even after leaving terminal mode if both
`terminal_enter` and (indirectly) `showmode` call `ui_busy_start`, as there is
only one matching call to `ui_busy_stop` after leaving terminal mode.

Solution: let `terminal_enter` handle setting the initial visibility of the
cursor before calling `showmode`.

Closes #32456.

This simple solution assumes it isn't possible for e.g. `os_breakcheck` to be
called indirectly by something else before `terminal_enter` initially handles
cursor visibility and after it restores it, which I think is true.

test/functional/terminal/cursor_spec.lua

@@ -357,6 +357,32 @@ describe(':terminal cursor', function()
 
     eq(error_hl_id, screen._mode_info[terminal_mode_idx].hl_id)
   end)
+
+  it('restores visibility on TermLeave #32456', function()
+    skip(is_os('win'), '#31587')
+    feed([[<C-\><C-N>]]) -- Exit terminal mode
+    screen:expect([[
+      tty ready                                         |
+      ^                                                  |
+                                                        |*5
+    ]])
+
+    tt.hide_cursor()
+    -- :startinsert repros the issue more reliably than feed('i')
+    command('mode | startinsert')
+    screen:expect([[
+      tty ready                                         |
+                                                        |*5
+      {3:-- TERMINAL --}                                    |
+    ]])
+
+    feed([[<C-\><C-N>]]) -- Exit terminal mode
+    screen:expect([[
+      tty ready                                         |
+      ^                                                  |
+                                                        |*5
+    ]])
+  end)
 end)
 
 describe('buffer cursor position is correct in terminal without number column', function()