Fix ContextVar LookupError in cross-context usage

[Rajat-Ahuja1997]

Summary

Fixes a regression introduced in v7.4.2 that causes LookupError when using cashews with FastAPI's
lifespan pattern or other cross-context scenarios.

Problem

Version 7.4.2 (commit a47f8ce) changed ControlMixin.__init__ from:

self.__disable: ContextVar[set[Command]] = ContextVar(str(id(self)), default=set())
to:
self.__disable: ContextVar[set[Command]] = ContextVar(str(id(self)))
self.__disable.set(set())

This breaks cross-context usage because:
- .set() only affects the current async context
- When cache operations run in different contexts (e.g., HTTP requests after setup in lifespan),
.get() raises LookupError because there's no value and no default

This is the same issue as #382, which was fixed for key_context.py in PR #383 (commit e22d2a7), but
the fix was not applied to ControlMixin.

Solution

Restore the default=set() parameter to the ContextVar constructor. This ensures .get() returns a
default value in all async contexts, not just where .set() was called.

Changes

- Restored default=set() parameter in cashews/backends/interface.py:183
- Removed redundant self.__disable.set(set()) call
- Added test_control_mixin_cross_context to verify the fix and prevent future regressions

Testing

- All existing tests pass (54/67 in test_disable_control.py, 13 fail due to missing optional
diskcache dependency)
- New test test_control_mixin_cross_context specifically tests cross-context behavior
- Verified test fails without the fix (LookupError) and passes with the fix

Related

- Fixes the same root cause as #382 / PR #383
- Affects users using FastAPI lifespan pattern or similar cross-context scenarios

[kthy]

@Krukov any chance this fix could be merged and we could get a point release? 🙏

[Krukov]

Sure, Sorry for the weak support, I have a difficult period in my life.

Thanks for MR !

[kthy]

No worries, hope you'll come out okay on the other side. Wishing you strength and luck!

[Rajat-Ahuja1997]

No worries, hope you'll come out okay on the other side. Wishing you strength and luck!

Agreed, hope things get better for you soon!