Fix libfabric transfer request lifecycle bugs (#1747)

## What?
Three fixes to the libfabric backend's transfer request lifecycle (Fixes
https://github.com/ai-dynamo/nixl/issues/1722)

  1. releaseReqH() now deletes the nixlLibfabricBackendH handle
2. Removed dead received_remote_writes_ field and unused <unordered_set>
include
3. CQ errors are now propagated to the transfer handle via the
completion callback, and the failed request is released back to the pool

## Why?
- Memory leak: Every postXfer() allocates a new nixlLibfabricBackendH
that was never freed
- Dead code: received_remote_writes_ was declared but never used in any
.cpp file
- CQ error hang: When fi_cq_read returned an error, the failed request
was never released to the pool and the owning handle was never notified
— causing transfers to hang indefinitely (and leaking pool entries)

## How?
Memory leak fix: Added delete static_cast<nixlLibfabricBackendH
*>(handle) in releaseReqH()

CQ error propagation: Changed completion_callback signature from void()
to void(nixl_status_t). On CQ error, the callback is invoked with
NIXL_ERR_BACKEND — this increments the completion counter (so
is_completed() eventually becomes true) and sets an error flag on the
handle. On success, NIXL_SUCCESS is passed as before.
checkXfer() waits for all requests to resolve before returning status.
This avoids premature cancellation of in-flight requests on other rails
— following the same drain-then-report pattern used by the UCX backend.


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

* **Bug Fixes**
* Transfer failures are now recorded and reported promptly, preventing
false successes and stuck requests.
* Completion errors now trigger failure notifications and reliably
release affected requests.
* Failed transfers no longer continue through success-only completion
handling.

* **API Changes**
* Transfer and control-message completion callbacks now provide explicit
status results.

* **Chores**
* Improved request cleanup and removed redundant transfer-tracking
state.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: Adit Ranadive <aranadive@nvidia.com>