Stabilize exec-server filesystem tests in CI (#17671)

## Summary\n- add an exec-server package-local test helper binary that
can run exec-server and fs-helper flows\n- route exec-server filesystem
tests through that helper instead of cross-crate codex helper
binaries\n- stop relying on Bazel-only extra binary wiring for these
tests\n\n## Testing\n- not run (per repo guidance for codex changes)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/test-binary-support/lib.rs

@@ -0,0 +1,77 @@
+use std::path::Path;
+
+use codex_arg0::Arg0DispatchPaths;
+use codex_arg0::Arg0PathEntryGuard;
+use codex_arg0::arg0_dispatch;
+use tempfile::TempDir;
+
+pub struct TestBinaryDispatchGuard {
+    _codex_home: TempDir,
+    arg0: Arg0PathEntryGuard,
+    _previous_codex_home: Option<std::ffi::OsString>,
+}
+
+impl TestBinaryDispatchGuard {
+    pub fn paths(&self) -> &Arg0DispatchPaths {
+        self.arg0.paths()
+    }
+}
+
+pub enum TestBinaryDispatchMode {
+    DispatchArg0Only,
+    Skip,
+    InstallAliases,
+}
+
+pub fn configure_test_binary_dispatch<F>(
+    codex_home_prefix: &str,
+    classify: F,
+) -> Option<TestBinaryDispatchGuard>
+where
+    F: FnOnce(&str, Option<&str>) -> TestBinaryDispatchMode,
+{
+    let mut args = std::env::args_os();
+    let argv0 = args.next().unwrap_or_default();
+    let exe_name = Path::new(&argv0)
+        .file_name()
+        .and_then(|name| name.to_str())
+        .unwrap_or("");
+    let argv1 = args.next();
+    match classify(exe_name, argv1.as_deref().and_then(|arg| arg.to_str())) {
+        TestBinaryDispatchMode::DispatchArg0Only => {
+            let _ = arg0_dispatch();
+            None
+        }
+        TestBinaryDispatchMode::Skip => None,
+        TestBinaryDispatchMode::InstallAliases => {
+            let codex_home = match tempfile::Builder::new().prefix(codex_home_prefix).tempdir() {
+                Ok(codex_home) => codex_home,
+                Err(error) => panic!("failed to create test CODEX_HOME: {error}"),
+            };
+            let previous_codex_home = std::env::var_os("CODEX_HOME");
+            // Safety: this runs from a test ctor before test threads begin.
+            unsafe {
+                std::env::set_var("CODEX_HOME", codex_home.path());
+            }
+
+            let arg0 = match arg0_dispatch() {
+                Some(arg0) => arg0,
+                None => panic!("failed to configure arg0 dispatch aliases for test binary"),
+            };
+            match previous_codex_home.as_ref() {
+                Some(value) => unsafe {
+                    std::env::set_var("CODEX_HOME", value);
+                },
+                None => unsafe {
+                    std::env::remove_var("CODEX_HOME");
+                },
+            }
+
+            Some(TestBinaryDispatchGuard {
+                _codex_home: codex_home,
+                arg0,
+                _previous_codex_home: previous_codex_home,
+            })
+        }
+    }
+}