Run code mode exclusively through the standalone host (#36217)

## What changed

- Move the V8 implementation into a dedicated `codex-code-mode-runtime` crate used by `codex-code-mode-host`, removing the embedded runtime fallback from the Codex process.
- Resolve the host executable from the active installation layout and check its availability before selecting tools.
- Fall back to direct tools with a one-time warning when optional code mode is unavailable. Keep `code_mode_only` and `disable_in_process_fallback` configurations fail-closed.

## Testing

- Cover host discovery for standalone and package layouts, including missing hosts and symlinks.
- Verify direct-tool fallback, one-time warnings, and fail-closed code-mode-only behavior.

GitOrigin-RevId: 5aa3c6f1db148b2231fc24089a2ee0e2b00dbddb

codex-rs/code-mode/src/remote_session/connection.rs

@@ -71,15 +71,6 @@ pub(super) enum ConnectionError {
     Other(String),
 }
 
-impl ConnectionError {
-    pub(super) fn host_program_not_found(&self) -> bool {
-        matches!(
-            self,
-            Self::Spawn { error, .. } if error.kind() == io::ErrorKind::NotFound
-        )
-    }
-}
-
 impl fmt::Display for ConnectionError {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {