fix(online): add interval check to RollingStrategy to prevent premature task generation (#2045)

[anish-devgit]

Summary
Fixes #2045: RollingStrategy should not prepare tasks if interval < rolling_step

What I changed

• Modified qlib/workflow/online/strategy.py: Added a check in prepare_tasks to return specific empty list if the calculated time interval is less than self.rg.step.

How I tested
Local: Added tests/test_issue_2045.py which mocks RollingGen, TimeAdjuster, and OnlineToolR.

• Verified that when interval=5 and step=10, prepare_tasks returns [].
• Verified that when interval=15 and step=10, prepare_tasks returns generated tasks.

Fixes #2045

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