fix(workflow): stop rolling when the next test window leaves the calendar

[Divyansh Barodiya (Divyansh151005)]

Description

RollingGen.gen_following_tasks stops when the next test window starts after the last trading day.

TimeAdjuster.get returns None for an index of len(calendar). That is the first position after the last trading day. shift does not raise KeyError there (KeyError is only for an index greater than len(calendar)), so the next test start is None. The old check compared that start with test_end and raised:

TypeError: '>' not supported between instances of 'NoneType' and 'Timestamp'

A missing test start is now the end of rolling, and that window is not emitted. A window that still begins on a trading day is kept, including one that ends on the last trading day. If another segment's start has already fallen off the calendar, that segment stays None and the in-range test window is still emitted.

Motivation and Context

Fixes #1660

Rolling benchmark generation hit this at the right edge of the calendar. The reporter's None guard is the right stop condition for the test segment. shift is left as it is so a train or valid segment can pass the end of the calendar without dropping a test window that is still valid.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

test_all_pipeline.py trains a CSI300 LightGBM workflow and needs the market dataset. These tests use a synthetic calendar and do not load market data.

$ python -m pytest tests/rolling_tests/test_rolling_calendar_boundary.py -q --tb=line
.....                                                                    [100%]
5 passed in 2.68s

flake8 on the changed Python files, with the ignore list from the developer guide, reported no issues. black -l 120 left the files unchanged.

Screenshots of Test Results (if appropriate):

1. Pipeline test: not run. It needs the CSI300 dataset and a model fit.
2. Your own tests: tests/rolling_tests/test_rolling_calendar_boundary.py, 5 passed.

Types of changes

• Fix bugs
• Add new feature
• Update documentation

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