bugfix: set up kv cache compleition guard to prevent kv cache transfer error.

[phantomlei3]

Description

In PD serving mode, Mooncake data transfer failures were not consistently propagated to the upper layers. Some worker exit paths could return
before signaling task completion, leaving the request in an incomplete state and potentially causing the scheduler to wait indefinitely.

Fix Approach

Introduce a completion guard to ensure that task completion is reported on every worker exit path, including early returns and failure branches.
Additionally, propagate Mooncake transfer errors consistently through the call chain so that failed transfers are detected and handled by the
scheduler instead of being silently ignored.

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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Reviewer Notes