Fix ImportError when importing SUPERVISOR_COMMS outside task context

[andreahlert]

Summary

Fixes #51816

SUPERVISOR_COMMS was declared as a bare type annotation (SUPERVISOR_COMMS: CommsDecoder[...]) in task_runner.py, which does not create an actual module attribute in Python. This caused ImportError when Variable.get() was called at the top level of a DAG file (outside task execution context), such as during dag.test().

Changes

• Initialize SUPERVISOR_COMMS to None so the import always succeeds. The existing check in ensure_secrets_backend_loaded() already handles is not None, so the fallback logic is unaffected.
• Add None guards before .send() calls in _set_variable(), _delete_variable(), and all ExecutionAPISecretsBackend methods to prevent AttributeError on None.
• Improve error messages when Variable.get() fails outside task context, suggesting alternatives (environment variables, Jinja templates, moving the call inside a task).
• Add tests for Variable.get/set/delete outside task execution context.

Files changed

File	Change
task_runner.py	SUPERVISOR_COMMS = None (was bare annotation)
context.py	None guards + enhanced error messages
execution_api.py	Defensive return None guards
test_variables.py	4 new tests for outside-task-context behavior

Test plan

• Existing test_variables.py tests pass (no regression)
• New TestVariableOutsideTaskContext tests pass
• Variable.get() with env var works outside task context
• Variable.get() without env var raises helpful error outside task context
• Variable.set() raises clear error outside task context
• Variable.delete() raises clear error outside task context

[SameerMesiah97]

This fix looks reasonable in isolation, but several provider tests appear to have relied on the previous permissive behavior when SUPERVISOR_COMMS was absent or not initialized. By normalizing it to None, execution-time paths (particularly around connection resolution) are now enforced and fail loudly, which is causing widespread provider CI failures. It seems those tests did not anticipate SUPERVISOR_COMMS being present but None and therefore entering these stricter code paths. It appears that provider tests might have to be loosened (or maybe connection resolution migh have to check for SUPERVISOR_COMMS) to accomodate this change but I am not going to make such a sweeping suggestion unilaterally.