Java SDK: Clear the XComs of a task instance at the start of each try

[FrankYang0529]

Follow-up for #74178 (review).

TaskRunner.runTask deletes the XComs when a try starts, as the Go and Python runtimes do. It sends a DeleteXCom message for each key after it finds the task in the bundle and before it instantiates the task class.

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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