fix(strategy): use get_risk_degree() in TopkDropoutStrategy for position sizing

[Akshay Kumar Hudedmani (akshaykumarhudedmani)]

Description

Fixes #2276. TopkDropoutStrategy.generate_trade_decision was sizing new buys using the raw attribute self.risk_degree instead of calling self.get_risk_degree(trade_step). This prevented subclasses from implementing market timing by overriding get_risk_degree(). This PR replaces self.risk_degree with self.get_risk_degree(trade_step).

Motivation and Context

Fixes #2276. Using the getter allows subclasses to dynamically adjust the risk degree over time, which is the documented way to implement market timing.

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.

Screenshots of Test Results (if appropriate):

Pipeline test passed successfully.

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[Akshay Kumar Hudedmani (akshaykumarhudedmani)]

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