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",