feat: pass helper executable paths via Arg0DispatchPaths (#12719)

## Why

`codex-rs/core/src/tools/runtimes/shell/unix_escalation.rs` previously
located `codex-execve-wrapper` by scanning `PATH` and sibling
directories. That lookup is brittle and can select the wrong binary when
the runtime environment differs from startup assumptions.

We already pass `codex-linux-sandbox` from `codex-arg0`;
`codex-execve-wrapper` should use the same startup-driven path plumbing.

## What changed

- Introduced `Arg0DispatchPaths` in `codex-arg0` to carry both helper
executable paths:
  - `codex_linux_sandbox_exe`
  - `main_execve_wrapper_exe`
- Updated `arg0_dispatch_or_else()` to pass `Arg0DispatchPaths` to
top-level binaries and preserve helper paths created in
`prepend_path_entry_for_codex_aliases()`.
- Threaded `Arg0DispatchPaths` through entrypoints in `cli`, `exec`,
`tui`, `app-server`, and `mcp-server`.
- Added `main_execve_wrapper_exe` to core configuration plumbing
(`Config`, `ConfigOverrides`, and `SessionServices`).
- Updated zsh-fork shell escalation to consume the configured
`main_execve_wrapper_exe` and removed path-sniffing fallback logic.
- Updated app-server config reload paths so reloaded configs keep the
same startup-provided helper executable paths.

## References

- [`Arg0DispatchPaths`
definition](https://github.com/openai/codex/blob/e355b43d5c2a771f045296a6deae10d7c9c36ec6/codex-rs/arg0/src/lib.rs#L20-L24)
- [`arg0_dispatch_or_else()` forwarding both
paths](https://github.com/openai/codex/blob/e355b43d5c2a771f045296a6deae10d7c9c36ec6/codex-rs/arg0/src/lib.rs#L145-L176)
- [zsh-fork escalation using configured wrapper
path](https://github.com/openai/codex/blob/e355b43d5c2a771f045296a6deae10d7c9c36ec6/codex-rs/core/src/tools/runtimes/shell/unix_escalation.rs#L109-L150)

## Testing

- `cargo check -p codex-arg0 -p codex-core -p codex-exec -p codex-tui -p
codex-mcp-server -p codex-app-server`
- `cargo test -p codex-arg0`
- `cargo test -p codex-core tools::runtimes::shell::unix_escalation:: --
--nocapture`

codex-rs/core/src/codex.rs

@@ -1331,6 +1331,7 @@ impl Session {
                 config.background_terminal_max_timeout,
             ),
             shell_zsh_path: config.zsh_path.clone(),
+            main_execve_wrapper_exe: config.main_execve_wrapper_exe.clone(),
             analytics_events_client: AnalyticsEventsClient::new(
                 Arc::clone(&config),
                 Arc::clone(&auth_manager),
@@ -8201,6 +8202,7 @@ mod tests {
                 config.background_terminal_max_timeout,
             ),
             shell_zsh_path: None,
+            main_execve_wrapper_exe: config.main_execve_wrapper_exe.clone(),
             analytics_events_client: AnalyticsEventsClient::new(
                 Arc::clone(&config),
                 Arc::clone(&auth_manager),
@@ -8356,6 +8358,7 @@ mod tests {
                 config.background_terminal_max_timeout,
             ),
             shell_zsh_path: None,
+            main_execve_wrapper_exe: config.main_execve_wrapper_exe.clone(),
             analytics_events_client: AnalyticsEventsClient::new(
                 Arc::clone(&config),
                 Arc::clone(&auth_manager),