[Bugfix][TENT] Unpin remote stage buffers after late completion (#3494)

* [TENT] Add regression test for late remote cleanup

Reproduce the remote stage-buffer leak when a Delegate RPC completes after ProxyManager destruction.

Signed-off-by: Ruiqing Feng <ruiqing.feng@broadcom.com>

* [Bugfix][TENT] Unpin remote stage buffers after late completion

Transfer cleanup ownership to unresolved remote operations during ProxyManager destruction and
asynchronously unpin their buffers after Delegate completion.

Signed-off-by: Ruiqing Feng <ruiqing.feng@broadcom.com>

* [TENT] Add regression test for unconfirmed remote completion

Signed-off-by: Ruiqing Feng <ruiqing.feng@broadcom.com>

* [Bugfix][TENT] Require confirmed Delegate completion before Unpin

Signed-off-by: Ruiqing Feng <ruiqing.feng@broadcom.com>

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Signed-off-by: Ruiqing Feng <ruiqing.feng@broadcom.com>