[Bugfix] Remove failed transfer from inflight on NIXL poll exception (#49528)

[jiangxt2]

Purpose

Fixes #49528: when NixlTransport.poll() catches an exception from
check_xfer_state(), the transfer was left in _inflight and retried
every poll cycle, producing an infinite loop of warnings.

The fix appends the failed transfer ID to failed_ids so the existing
post-loop cleanup removes it from _inflight and attempts to release the
handle. Both poll() and _release_handles() warning logs now include
exc_info=True for full traceback diagnosis.

Duplicate check

Searched open PRs referencing _inflight cleanup and NIXL poll
exception handling; no open PR fixes the inflight leak when
check_xfer_state() raises.

Test Plan

# Lint
ruff check vllm/v1/kv_offload/tiering/p2p/data/nixl.py \
         tests/v1/kv_offload/tiering/p2p/test_data_transport.py

# Unit tests
.venv/bin/python -m pytest \
  tests/v1/kv_offload/tiering/p2p/test_data_transport.py -v

Test Result

ruff: All checks passed!
pytest: 27 passed in 2.39s

---

This PR was prepared with AI assistance (Claude). A human reviewed every
changed line, ran the tests, and takes responsibility for the change.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[jiangxt2]

Could a maintainer please add the ready label to trigger CI? This is my first PR to the repo. Thanks!