Prevent ConPTY DLL loading from the current directory (#39262)

## Why

The Windows PTY implementation probed `conpty.dll` by bare name. Windows could
resolve that lookup to a DLL in the process current directory and load it when
creating a pseudoconsole.

## What changed

Resolve the ConPTY entry points from `kernel32.dll` without probing
`conpty.dll`.

## Testing

Add a Windows regression test that places a compatible `conpty.dll` in the
current directory, creates a ConPTY, and verifies that the DLL was not loaded.

GitOrigin-RevId: be78974362a99dce0c18186cf3e7bbd86a8de73c

codex-rs/utils/pty/Cargo.toml

@@ -24,6 +24,7 @@ winapi = { version = "0.3.9", features = [
     "handleapi",
     "jobapi",
     "jobapi2",
+    "libloaderapi",
     "minwinbase",
     "processthreadsapi",
     "synchapi",

codex-rs/utils/pty/src/win/psuedocon.rs

@@ -85,20 +85,10 @@ shared_library!(Ntdll,
     ) -> NTSTATUS,
 );
 
-fn load_conpty() -> ConPtyFuncs {
-    let kernel = ConPtyFuncs::open(Path::new("kernel32.dll")).expect(
+lazy_static! {
+    static ref CONPTY: ConPtyFuncs = ConPtyFuncs::open(Path::new("kernel32.dll")).expect(
         "this system does not support conpty.  Windows 10 October 2018 or newer is required",
     );
-
-    if let Ok(sideloaded) = ConPtyFuncs::open(Path::new("conpty.dll")) {
-        sideloaded
-    } else {
-        kernel
-    }
-}
-
-lazy_static! {
-    static ref CONPTY: ConPtyFuncs = load_conpty();
 }
 
 pub fn conpty_supported() -> bool {

codex-rs/utils/pty/tests/conpty_search_path.rs

@@ -0,0 +1,70 @@
+#![cfg(windows)]
+
+use codex_utils_pty::RawConPty;
+use std::ffi::OsStr;
+use std::fs;
+use std::os::windows::ffi::OsStrExt;
+use std::path::Path;
+use std::path::PathBuf;
+use winapi::um::libloaderapi::GetModuleHandleW;
+
+struct CurrentDirGuard {
+    original: PathBuf,
+}
+
+impl CurrentDirGuard {
+    fn enter(path: &Path) -> std::io::Result<Self> {
+        let original = std::env::current_dir()?;
+        std::env::set_current_dir(path)?;
+        Ok(Self { original })
+    }
+}
+
+impl Drop for CurrentDirGuard {
+    fn drop(&mut self) {
+        let _ = std::env::set_current_dir(&self.original);
+    }
+}
+
+#[test]
+fn conpty_ignores_dll_in_current_directory() -> anyhow::Result<()> {
+    let temp_dir = std::env::temp_dir().join(format!(
+        "codex-conpty-search-path-{}-{}",
+        std::process::id(),
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)?
+            .as_nanos()
+    ));
+    fs::create_dir(&temp_dir)?;
+
+    let system_root =
+        std::env::var_os("SystemRoot").ok_or_else(|| anyhow::anyhow!("SystemRoot is not set"))?;
+    let candidate = temp_dir.join("conpty.dll");
+    // A kernel32 copy exports the ConPTY functions, so the old bare-name probe
+    // would successfully load and retain this repository-controlled file.
+    fs::copy(
+        PathBuf::from(system_root)
+            .join("System32")
+            .join("kernel32.dll"),
+        &candidate,
+    )?;
+
+    {
+        let _current_dir = CurrentDirGuard::enter(&temp_dir)?;
+        drop(RawConPty::new(/*cols*/ 80, /*rows*/ 24)?);
+
+        let conpty_dll = OsStr::new("conpty.dll")
+            .encode_wide()
+            .chain(std::iter::once(0))
+            .collect::<Vec<_>>();
+        let module = unsafe { GetModuleHandleW(conpty_dll.as_ptr()) };
+        assert!(
+            module.is_null(),
+            "ConPTY loaded conpty.dll from the process current directory"
+        );
+    }
+
+    fs::remove_file(candidate)?;
+    fs::remove_dir(temp_dir)?;
+    Ok(())
+}