Expand Vim change commands and add character replacement (#39661)

## What changed

- Add the configurable `vim_normal.replace_char` action, bound to `r` by default, to replace the grapheme under the cursor while remaining in normal mode.
- Support change-operator motions such as `cw`, `c$`, `cj`, and `ck`, plus the repeated `cc` command for changing the current line.
- Let `Esc` cancel a pending replacement before it reaches composer-level handling.
- Preserve existing custom Vim bindings and chord prefixes when introducing the new default.

## Testing

- Cover replacement, grapheme boundaries, remapping and unbinding, change motions, cancellation, keymap conflicts, and keymap picker snapshots.

GitOrigin-RevId: 98feb4eeb57142c37adb73abfe58d1f703b9270d

codex-rs/config/src/tui_keymap.rs

@@ -231,6 +231,8 @@ pub struct TuiVimNormalKeymap {
     pub move_line_end: Option<KeybindingsSpec>,
     /// Delete character under cursor (`x`).
     pub delete_char: Option<KeybindingsSpec>,
+    /// Replace the character under the cursor (`r`).
+    pub replace_char: Option<KeybindingsSpec>,
     /// Delete character under cursor and enter insert mode (`s`).
     pub substitute_char: Option<KeybindingsSpec>,
     /// Delete from cursor to end of line (`D`).