Fix TopkDropoutStrategy dynamic risk sizing

[Kevin-Li-2025]

Summary

Fixes #2276.

TopkDropoutStrategy exposes get_risk_degree(trade_step) so subclasses can implement dynamic market-timing / position-sizing logic, but the buy budget used the fixed self.risk_degree field directly. This makes dynamic risk overrides ineffective for new buys.

This PR routes the buy budget through get_risk_degree(trade_step) and adds a focused regression test that keeps self.risk_degree = 0.95 while overriding get_risk_degree() to return 0.25; the generated buy order must use the dynamic 25% budget.

Tests

• PYTHONPYCACHEPREFIX=/private/tmp/qlib-pycache /usr/bin/python3 -m py_compile qlib/contrib/strategy/signal_strategy.py tests/backtest/test_topk_dropout_strategy.py
• git diff --check
• PYTHONPYCACHEPREFIX=/private/tmp/qlib-pycache /opt/homebrew/bin/python3.11 - <<'PY' ... isolated harness that stubs Qlib package-level imports, loads qlib/contrib/strategy/signal_strategy.py, and verifies dynamic risk produces a 25-share order from 1000 cash at price 10

Note: direct local pytest -q tests/backtest/test_topk_dropout_strategy.py is blocked in this unbuilt source tree by missing compiled extensions (qlib.data._libs.rolling). I attempted setup.py build_ext --inplace; this checkout lacks generated C++ sources and needs the project Cython build path first. The added test is a normal repository test and should run in CI after the extension build step.

[Kevin-Li-2025]

Quick follow-up on scope: this is intentionally a small fix for #2276. TopkDropoutStrategy already exposes get_risk_degree(trade_step) for dynamic risk / market timing, and WeightStrategyBase already routes sizing through that hook. This PR makes the TopK dropout buy budget use the same hook instead of the fixed self.risk_degree field.

I added a focused regression test where self.risk_degree remains 0.95 but an override returns 0.25, and the generated buy order is sized from the dynamic 25% budget. Happy to adjust the test style if maintainers prefer this covered in an existing backtest test file.