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/app-server/src/message_processor.rs

@@ -1,5 +1,4 @@
 use std::collections::HashSet;
-use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::RwLock;
 use std::sync::atomic::AtomicBool;
@@ -32,6 +31,7 @@ use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::ServerNotification;
 use codex_app_server_protocol::ServerRequestPayload;
 use codex_app_server_protocol::experimental_required_message;
+use codex_arg0::Arg0DispatchPaths;
 use codex_core::AuthManager;
 use codex_core::ThreadManager;
 use codex_core::auth::ExternalAuthRefreshContext;
@@ -139,7 +139,7 @@ pub(crate) struct ConnectionSessionState {
 
 pub(crate) struct MessageProcessorArgs {
     pub(crate) outgoing: Arc<OutgoingMessageSender>,
-    pub(crate) codex_linux_sandbox_exe: Option<PathBuf>,
+    pub(crate) arg0_paths: Arg0DispatchPaths,
     pub(crate) config: Arc<Config>,
     pub(crate) single_client_mode: bool,
     pub(crate) cli_overrides: Vec<(String, TomlValue)>,
@@ -155,7 +155,7 @@ impl MessageProcessor {
     pub(crate) fn new(args: MessageProcessorArgs) -> Self {
         let MessageProcessorArgs {
             outgoing,
-            codex_linux_sandbox_exe,
+            arg0_paths,
             config,
             single_client_mode,
             cli_overrides,
@@ -184,7 +184,7 @@ impl MessageProcessor {
             auth_manager,
             thread_manager,
             outgoing: outgoing.clone(),
-            codex_linux_sandbox_exe,
+            arg0_paths,
             config: Arc::clone(&config),
             cli_overrides: cli_overrides.clone(),
             cloud_requirements: cloud_requirements.clone(),