helix: Fix a cursor position in helix_select mode` (#57610)

Closes #56493

In Helix select mode, pressing `a` after a selection (e.g. `v a`) placed
the cursor one column too far to the right.

Bound `a` to `vim::HelixAppend` in the `helix_select` keymap so it
matches the behavior in `helix_normal`, and added a regression test for
the `v a` case.

Release Notes:

- Fixed cursor placement after pressing `a` in Helix select mode.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

assets/keymaps/vim.json

@@ -338,7 +338,7 @@
       "ctrl-x": "vim::Decrement",
       "shift-j": "vim::JoinLines",
       "i": "vim::InsertBefore",
-      "a": "vim::InsertAfter",
+      "a": "vim::HelixAppend",
       "o": "vim::InsertLineBelow",
       "shift-o": "vim::InsertLineAbove",
       "p": "vim::Paste",

crates/vim/src/helix.rs

@@ -2572,6 +2572,16 @@ mod test {
         assert_eq!(cx.mode(), Mode::HelixNormal);
     }
 
+    #[gpui::test]
+    async fn test_helix_select_append(cx: &mut gpui::TestAppContext) {
+        let mut cx = VimTestContext::new(cx, true).await;
+        cx.enable_helix();
+
+        cx.set_state("aˇbcd", Mode::HelixNormal);
+        cx.simulate_keystrokes("v a");
+        cx.assert_state("abˇcd", Mode::Insert);
+    }
+
     #[gpui::test]
     async fn test_goto_last_modification(cx: &mut gpui::TestAppContext) {
         let mut cx = VimTestContext::new(cx, true).await;