UCT: outstanding purge implementation (#11668)

* UCT: rebuild failover implementation on finalized API

Rebuild the transport implementation on top of the API-only
failover_ext_uct branch:
- align naming to the finalized "purge" API (outstanding purge callback,
  params and field mask), dropping the intermediate "extract"/flags shape
- switch the mlx5 external plugin path from qp_query to ep_query, with RX
  token derivation performed by the iface query
- re-add the uct_ep_outstanding_purge()/uct_ep_failover_enable() prototypes
  and the uct_ib_mlx5_ext_ep_outstanding_purge() dispatch wrapper
- update gtests (test_ib_mlx5_ext, test_uct_outstanding_purge) accordingly

* UCT: cleanup

* UCT: keep plugin impl only

* UCT/IB/MLX5: address comments

* UCT: cleanup and format

* GTEST/UCT/IB: address comments

* UCT/IB/MLX5: address comments

* UCT/IB/MLX5: address comments

* UCT: address comments

* GTEST/UCT: address comments

* UCT/IB/MLX5: address comments

* UCT: address comments

* GTEST: reduce gtest cases

* GTEST/UCT/IB: update gtest

* GTEST/UCT/IB: fix coverity

* GTEST/UCT/IB: fix build

* UCT/IB/MLX5: address comments

* GTEST/UCT/IB: fix gtest

* UCT: address comments

* UCT/IB/MLX5: fix format

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Co-authored-by: Zihao Zhao <zizhao@nvidia.com>