enhance: add cluster-wide read task queue clearing (#50090)

Related to #50089

Add a Proxy management entry point that routes through MixCoord to clear
queued read tasks across all Proxy and QueryNode instances. Queued tasks
are failed with canceled semantics and per-component counts are returned
so operators can recover from read-task backlog without attempting
unsafe active-task preemption.

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Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>