TS SDK: delete a task instance's existing XComs when a try starts

[FrankYang0529]

Follow-up for #74178 (review).

handleTask sends a DeleteXCom for each listed key after it finds the handler and before it binds arguments. This matches the Python task runner, which deletes the listed XComs before it renders templates.

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

• Yes - Claude Code

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