Support CXOTIME_NOW environment variable

[taldcroft]

Description

This fixes cheta to support the CXOTIME_NOW environment variable and use it to mock the current time if defined.

The original code logic ignored that DateTime(None) returns the current time, so this PR fixes that.

Requires

• Support cxotime now sentinel and env var chandra_time#59

Interface impacts

Testing

Unit tests

• Mac

(ska3-flight-2025.0rc2) ➜  cheta git:(support-cxotime-now) git rev-parse --short HEAD
7488498
(ska3-flight-2025.0rc2) ➜  cheta git:(support-cxotime-now) pytest                    
=============================================== test session starts ================================================
platform darwin -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0
rootdir: /Volumes/git
configfile: pytest.ini
plugins: doctestplus-1.3.0, anyio-4.7.0, timeout-2.3.1
collected 175 items                                                                                                

cheta/tests/test_comps.py ............................................................                       [ 34%]
cheta/tests/test_data_source.py .........                                                                    [ 39%]
cheta/tests/test_fetch.py .................................                                                  [ 58%]
cheta/tests/test_intervals.py .........................                                                      [ 72%]
cheta/tests/test_orbit.py .                                                                                  [ 73%]
cheta/tests/test_remote_access.py ......                                                                     [ 76%]
cheta/tests/test_sync.py ........                                                                            [ 81%]
cheta/tests/test_units.py ...........                                                                        [ 87%]
cheta/tests/test_units_reversed.py ...........                                                               [ 93%]
cheta/tests/test_utils.py ...........                                                                        [100%]

========================================= 175 passed in 128.31s (0:02:08) ==========================================

Independent check of unit tests by Jean

• osx arm64

(2025.1rc1) flame:cheta jean$ pytest
============================================================= test session starts =============================================================
platform darwin -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0
rootdir: /Users/jean/git
configfile: pytest.ini
plugins: anyio-4.7.0, timeout-2.3.1
collected 175 items                                                                                                                           

cheta/tests/test_comps.py ............................................................                                                  [ 34%]
cheta/tests/test_data_source.py .........                                                                                               [ 39%]
cheta/tests/test_fetch.py .................................                                                                             [ 58%]
cheta/tests/test_intervals.py .........................                                                                                 [ 72%]
cheta/tests/test_orbit.py .                                                                                                             [ 73%]
cheta/tests/test_remote_access.py ......                                                                                                [ 76%]
cheta/tests/test_sync.py ........                                                                                                       [ 81%]
cheta/tests/test_units.py ...........                                                                                                   [ 87%]
cheta/tests/test_units_reversed.py ...........                                                                                          [ 93%]
cheta/tests/test_utils.py ...........                                                                                                   [100%]

============================================================== warnings summary ===============================================================
cheta/cheta/tests/test_comps.py::test_cmd_states
  /Users/jean/miniforge3/envs/2025.1rc1/lib/python3.12/site-packages/setuptools_scm/git.py:312: UserWarning: git archive did not support describe output
    warnings.warn("git archive did not support describe output")

cheta/cheta/tests/test_intervals.py::test_fetch_MSID_intervals
  /Users/jean/miniforge3/envs/2025.1rc1/lib/python3.12/site-packages/django/utils/encoding.py:266: DeprecationWarning: 'locale.getdefaultlocale' is deprecated and slated for removal in Python 3.15. Use setlocale(), getencoding() and getlocale() instead.
    encoding = locale.getdefaultlocale()[1] or 'ascii'

cheta/cheta/tests/test_intervals.py::test_fetch_MSID_intervals
  /Users/jean/miniforge3/envs/2025.1rc1/lib/python3.12/site-packages/django/http/request.py:1: DeprecationWarning: 'cgi' is deprecated and slated for removal in Python 3.13
    import cgi

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================================================= 175 passed, 3 warnings in 86.78s (0:01:26

Functional tests

No functional testing.