Fix flaky EngineProcessor time filter tests

[senahazir]

This fix addresses the flaky tests in engine_processor_test.py as explained in #7851.
The tests are fixed by mocking the datetime.datetime class with a subclass
_FrozenDateTime to make datetime.datetime.now() return a constant.

I reproduced the issue by adding import time; time.sleep(1) to both test_get_schedule_time_filter_behavior and test_list_reservations_time_filter_behavior and confirmed that the fix works for both cases (though I removed the sleeps for this pr).

Fixes #7851

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.63%. Comparing base (b44b862) to head (b63dda1).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8058      +/-   ##
==========================================
- Coverage   99.63%   99.63%   -0.01%     
==========================================
  Files        1110     1110              
  Lines       99794    99954     +160     
==========================================
+ Hits        99433    99591     +158     
- Misses        361      363       +2     

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[pavoljuhas]

I think we can fix this without changing engine_processor.py.

Following the idea in #7851 (comment), here is a datetime.datetime subclass that can be added to engine_processor_test.py

class _FrozenDateTime(datetime.datetime):

    _stock_datetime = datetime.datetime

    @classmethod
    @functools.cache
    def now(cls, tz=None):
        return cls._stock_datetime.now(tz)

The affected test functions can be then decorated with

@mock.patch('datetime.datetime', _FrozenDateTime)

which would replacedatetime.datetime for their execution.

[senahazir]

Updated the PR with the requested changes.

[pavoljuhas]

LGTM after pushing some code tweaks and updating PR description.
(The description is used for git commit message so we want it to
be in sync with changes)

Thank you for the fix!