sandboxing: use OsString for SandboxCommand.program (#15897)

## Why

`SandboxCommand.program` represents an executable path, but keeping it
as `String` forced path-backed callers to run `to_string_lossy()` before
the sandbox layer ever touched the command. That loses fidelity earlier
than necessary and adds avoidable conversions in runtimes that already
have a `PathBuf`.

## What changed

- Changed `SandboxCommand.program` to `OsString`.
- Updated `SandboxManager::transform` to keep the program and argv in
`OsString` form until the `SandboxExecRequest` conversion boundary.
- Switched the path-backed `apply_patch` and `js_repl` runtimes to pass
`into_os_string()` instead of `to_string_lossy()`.
- Updated the remaining string-backed builders and tests to match the
new type while preserving the existing Linux helper `arg0` behavior.

## Verification

- `cargo test -p codex-sandboxing`
- `just argument-comment-lint -p codex-core -p codex-sandboxing`
- `cargo test -p codex-core` currently fails in unrelated existing
config tests: `config::tests::approvals_reviewer_*` and
`config::tests::smart_approvals_alias_*`

codex-rs/sandboxing/src/manager_tests.rs

@@ -76,7 +76,7 @@ fn transform_preserves_unrestricted_file_system_policy_for_restricted_network()
     let exec_request = manager
         .transform(SandboxTransformRequest {
             command: SandboxCommand {
-                program: "true".to_string(),
+                program: "true".into(),
                 args: Vec::new(),
                 cwd: cwd.clone(),
                 env: HashMap::new(),
@@ -122,7 +122,7 @@ fn transform_additional_permissions_enable_network_for_external_sandbox() {
     let exec_request = manager
         .transform(SandboxTransformRequest {
             command: SandboxCommand {
-                program: "true".to_string(),
+                program: "true".into(),
                 args: Vec::new(),
                 cwd: cwd.clone(),
                 env: HashMap::new(),
@@ -181,7 +181,7 @@ fn transform_additional_permissions_preserves_denied_entries() {
     let exec_request = manager
         .transform(SandboxTransformRequest {
             command: SandboxCommand {
-                program: "true".to_string(),
+                program: "true".into(),
                 args: Vec::new(),
                 cwd: cwd.clone(),
                 env: HashMap::new(),
@@ -259,7 +259,7 @@ fn transform_linux_seccomp_request(
     manager
         .transform(SandboxTransformRequest {
             command: SandboxCommand {
-                program: "true".to_string(),
+                program: "true".into(),
                 args: Vec::new(),
                 cwd: cwd.clone(),
                 env: HashMap::new(),