airflowctl: fix flaky xcom add test by using unique keys per test run

[Dev-iL]

• Fix flaky test_airflowctl_commands[xcom add] test that intermittently fails with The XCom with key: 'test_xcom_key' with mentioned task instance already exists.
• The hardcoded test_xcom_key could collide with leftover state from a previous failed run where xcom delete never executed (e.g., CI timeout or earlier test failure)
• Derive a unique xcom key per parametrize set from the already-randomized date_param, so both the debug-mode and skip-keyring test sets use distinct keys
• Add a regression test (test_hardcoded_xcom_key_would_collide) that demonstrates the old hardcoded key produces identical commands across sets while the derived keys always differ.

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