[flink] Report pending finished tiering tables on enumerator shutdown

[yangshangqing95]

Purpose

Fix #4702

TieringSourceEnumerator may lose completed tiering reports if it is closed before a scheduled asynchronous heartbeat executes.

Previously, close() only reported failed tables, leaving pending entries in finishedTables unreported. This could leave the Coordinator treating an already completed table as still being tiered, delaying subsequent tiering rounds.

Brief change log

• Report both pending finished and failed tables through the existing heartbeat mechanism during enumerator shutdown.
• Ensure heartbeat failures do not prevent resource cleanup.

Tests

Added two regression tests in TieringSourceEnumeratorTest:

• testCloseReportsFinishedAndRunningTablesBeforePendingHeartbeat: Verifies that finished and running tables are correctly reported when the enumerator closes before the pending heartbeat executes.
• testCloseReleasesRpcClientWhenHeartbeatFails: Verifies that the RPC client is still closed when the shutdown heartbeat fails.

API and Format

No changes.

Documentation

Not applicable.