Fix XCom get_value outside task runner

[sdhilip200]

When BaseXCom.get_value() is called outside a task-runner context, such as from plugin extra links resolved by the API server, BaseXCom.get_one() attempted to import SUPERVISOR_COMMS and raised ImportError before it could read the stored XCom value.

This changes sync BaseXCom.get_one() to keep using supervisor comms when they are available, and to fall back to reading the matching XCom row from the metadata database when no supervisor comms are present. The fallback uses XComModel.deserialize_value() so DB-backed values are decoded consistently with the model path.

Closes #59093

Tests:

• uv run pytest airflow-core/tests/unit/models/test_xcom.py::TestXComGet::test_basexcom_get_value_reads_from_db_without_supervisor_comms
• uv run pytest airflow-core/tests/unit/models/test_xcom.py::TestXComGet
• uv run pytest task-sdk/tests/task_sdk/bases/test_xcom.py
• uv run pytest airflow-core/tests/unit/models/test_xcom.py
• uv run ruff check task-sdk/src/airflow/sdk/bases/xcom.py airflow-core/tests/unit/models/test_xcom.py
• uv run ruff format --check task-sdk/src/airflow/sdk/bases/xcom.py airflow-core/tests/unit/models/test_xcom.py

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• Yes: OpenAI Codex

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