fix: stop flink job error by using flink cancel command before process kill

[shenshichao163-oss]

What this PR does

Fixes the issue where stopping Flink tasks from the DolphinScheduler page fails to send proper kill signals, causing errors in worker node logs.

Root Cause

When a user clicks "Stop" on a Flink task, DolphinScheduler uses the generic AbstractCommandExecutor.cancelApplication() method, which sends OS-level kill signals (SIGINT → SIGTERM → SIGKILL) to the process tree. This approach does not properly stop the Flink job — it kills the shell process but fails to gracefully cancel the Flink application, leading to:

• Error logs in worker nodes
• Flink jobs not being properly terminated
• Resource leaks in the YARN/Kubernetes cluster

Fix

Override cancelApplication() in FlinkTask to:

1. First attempt graceful cancellation using the flink cancel <appId> command (via FlinkArgsUtils.buildCancelCommandLine()). This method already existed in the codebase but was never called.
2. Fall back to process-level kill (the parent class implementation) if the Flink cancel command fails, times out (30s), or no appIds are available.

Changes

• FlinkTask.java: Added cancelApplication() override that tries flink cancel first, then falls back to super.cancelApplication()
• Added imports: TaskException, TaskCallBack

Issue Link

Fixes #16789

Before

Stopping a Flink task from the page → AbstractCommandExecutor.cancelApplication() → ProcessUtils.kill() (SIGINT/SIGTERM/SIGKILL) → Flink job not properly cancelled, worker logs show errors

After

Stopping a Flink task from the page → FlinkTask.cancelApplication() → flink cancel <appId> (graceful) → if fails → super.cancelApplication() (process kill as fallback)

Checklist

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked that this modification does not cause CI to fail.
• I will sign the Apache CLA if required.
• The code follows the projects coding style.

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