Resolves https://github.com/zed-industries/zed/issues/57522

This diff fixes `r` in Helix select mode. The keybinding already pushed
the replace operator in `helix_select`, but when the replacement
character was typed, the operator dispatch only handled `HelixNormal`,
so selected text in `HelixSelect` fell through and cleared the operator
without editing the buffer.

With this change, Helix select mode uses the same `helix_replace` path
as Helix normal mode. Multi-character selections now replace each
selected grapheme with the typed character and return to Helix normal
mode, matching the existing behaviour for Helix normal selections.

Release Notes:
- Fixed `r` not replacing multi-character selections in Helix select
mode.