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](e355b43d5c/codex-rs/arg0/src/lib.rs (L20-L24))
- [`arg0_dispatch_or_else()` forwarding both
paths](e355b43d5c/codex-rs/arg0/src/lib.rs (L145-L176))
- [zsh-fork escalation using configured wrapper
path](e355b43d5c/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/mcp-server/src/lib.rs

@@ -3,8 +3,8 @@
 
 use std::io::ErrorKind;
 use std::io::Result as IoResult;
-use std::path::PathBuf;
 
+use codex_arg0::Arg0DispatchPaths;
 use codex_core::config::Config;
 use codex_utils_cli::CliConfigOverrides;
 
@@ -49,7 +49,7 @@ const CHANNEL_CAPACITY: usize = 128;
 type IncomingMessage = JsonRpcMessage<ClientRequest, Value, ClientNotification>;
 
 pub async fn run_main(
-    codex_linux_sandbox_exe: Option<PathBuf>,
+    arg0_paths: Arg0DispatchPaths,
     cli_config_overrides: CliConfigOverrides,
 ) -> IoResult<()> {
     // Install a simple subscriber so `tracing` output is visible.  Users can
@@ -105,7 +105,7 @@ pub async fn run_main(
         let outgoing_message_sender = OutgoingMessageSender::new(outgoing_tx);
         let mut processor = MessageProcessor::new(
             outgoing_message_sender,
-            codex_linux_sandbox_exe,
+            arg0_paths,
             std::sync::Arc::new(config),
         );
         async move {