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/core/src/config/mod.rs

@@ -399,6 +399,11 @@ pub struct Config {
     /// When this program is invoked, arg0 will be set to `codex-linux-sandbox`.
     pub codex_linux_sandbox_exe: Option<PathBuf>,
 
+    /// Path to the `codex-execve-wrapper` executable used for shell
+    /// escalation. This cannot be set in the config file: it must be set in
+    /// code via [`ConfigOverrides`].
+    pub main_execve_wrapper_exe: Option<PathBuf>,
+
     /// Optional absolute path to the Node runtime used by `js_repl`.
     pub js_repl_node_path: Option<PathBuf>,
 
@@ -646,7 +651,8 @@ impl Config {
     /// designed to use [AskForApproval::Never] exclusively.
     ///
     /// Further, [ConfigOverrides] contains some options that are not supported
-    /// in [ConfigToml], such as `cwd` and `codex_linux_sandbox_exe`.
+    /// in [ConfigToml], such as `cwd`, `codex_linux_sandbox_exe`, and
+    /// `main_execve_wrapper_exe`.
     pub async fn load_with_cli_overrides_and_harness_overrides(
         cli_overrides: Vec<(String, TomlValue)>,
         harness_overrides: ConfigOverrides,
@@ -1536,6 +1542,7 @@ pub struct ConfigOverrides {
     pub model_provider: Option<String>,
     pub config_profile: Option<String>,
     pub codex_linux_sandbox_exe: Option<PathBuf>,
+    pub main_execve_wrapper_exe: Option<PathBuf>,
     pub js_repl_node_path: Option<PathBuf>,
     pub js_repl_node_module_dirs: Option<Vec<PathBuf>>,
     pub zsh_path: Option<PathBuf>,
@@ -1665,6 +1672,7 @@ impl Config {
             model_provider,
             config_profile: config_profile_key,
             codex_linux_sandbox_exe,
+            main_execve_wrapper_exe,
             js_repl_node_path: js_repl_node_path_override,
             js_repl_node_module_dirs: js_repl_node_module_dirs_override,
             zsh_path: zsh_path_override,
@@ -2151,6 +2159,7 @@ impl Config {
             ephemeral: ephemeral.unwrap_or_default(),
             file_opener: cfg.file_opener.unwrap_or(UriBasedFileOpener::VsCode),
             codex_linux_sandbox_exe,
+            main_execve_wrapper_exe,
             js_repl_node_path,
             js_repl_node_module_dirs,
             zsh_path,
@@ -4765,6 +4774,7 @@ model_verbosity = "high"
                 ephemeral: false,
                 file_opener: UriBasedFileOpener::VsCode,
                 codex_linux_sandbox_exe: None,
+                main_execve_wrapper_exe: None,
                 js_repl_node_path: None,
                 js_repl_node_module_dirs: Vec::new(),
                 zsh_path: None,
@@ -4891,6 +4901,7 @@ model_verbosity = "high"
             ephemeral: false,
             file_opener: UriBasedFileOpener::VsCode,
             codex_linux_sandbox_exe: None,
+            main_execve_wrapper_exe: None,
             js_repl_node_path: None,
             js_repl_node_module_dirs: Vec::new(),
             zsh_path: None,
@@ -5015,6 +5026,7 @@ model_verbosity = "high"
             ephemeral: false,
             file_opener: UriBasedFileOpener::VsCode,
             codex_linux_sandbox_exe: None,
+            main_execve_wrapper_exe: None,
             js_repl_node_path: None,
             js_repl_node_module_dirs: Vec::new(),
             zsh_path: None,
@@ -5125,6 +5137,7 @@ model_verbosity = "high"
             ephemeral: false,
             file_opener: UriBasedFileOpener::VsCode,
             codex_linux_sandbox_exe: None,
+            main_execve_wrapper_exe: None,
             js_repl_node_path: None,
             js_repl_node_module_dirs: Vec::new(),
             zsh_path: None,