bindings: free the dlist handle wrapper in release_xfer_dlist_handle (#1829)

## What?

`nixl_capi_prep_xfer_dlist` (`src/bindings/rust/wrapper.cpp`) allocates
the handle
wrapper with `new nixl_capi_xfer_dlist_handle_s`, but
`nixl_capi_release_xfer_dlist_handle` only released the **inner** handle
and never
freed the wrapper:

```diff
     try {
         nixl_status_t ret = agent->inner->releasedDlistH(dlist_handle->handle);
+        delete dlist_handle;
         return ret == NIXL_SUCCESS ? NIXL_CAPI_SUCCESS : NIXL_CAPI_ERROR_BACKEND;
     }
```

## Why?

`releasedDlistH()` deletes the inner `nixlDlistH`; the
`nixl_capi_xfer_dlist_handle_s`
wrapper that `prep` `new`'d is a **separate** heap allocation that was
never freed, so
each prep/release cycle leaks one wrapper. Rust's
`XferDlistHandle::drop` calls release
exactly once and never retries, so an unconditional `delete` here is the
sole, correct
teardown (no double-free).

### Reproduction

Extracted LeakSanitizer reproducer of the `new wrapper` /
release-frees-only-inner
shape (`-fsanitize=address,leak`):

```
buggy:  SUMMARY: AddressSanitizer: 8 byte(s) leaked in 1 allocation(s)
fixed:  no leak
```

## How (verification)

- Confirmed the before/after with the reproducer above.
- Compiled the modified `wrapper.cpp` **in-tree** with
`-Dsanitizer=address,undefined`
  (exit 0).

## Related Issues

None.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Fixed a handle cleanup issue so released transfer handles are now
properly freed after use, helping prevent memory leaks.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Eylon Krause <eylon1909@gmail.com>