Go SDK: delete the XComs of earlier tries before a task runs

[FrankYang0529]

Follow-up for #74075 (comment).

When a task instance starts a new try, Airflow keeps the XComs it already has and lists their keys in ti_context.xcom_keys_to_clear for the runtime to delete. The Python task runner deletes them before it renders templates. The Go runtime never read the field, so a retried Go task still saw the XComs of its previous try. With this change the Go runtime deletes them too, and the retried task no longer sees them.

• RunTask sends a DeleteXCom for each key after it finds the task and before it binds arguments or runs the task.
• If a delete fails, the task does not run. It ends with a RetryTask when should_retry is set and with a FAILED TaskState otherwise.
• dag.If no longer writes {"skipped": []} to skipmixin_key before the condition runs, since the delete now removes the list from an earlier try.

---

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

• Yes - Claude Code

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.