fix(model): keep zero-score search results

[he-yufeng]

Summary

• keep the first completed CF/CTR model-search trial even when its score is zero
• add regression coverage for zero-score CF and CTR searches

Why

With very small datasets, every optimization trial can legitimately score 0. The previous comparison only saved a result when the score was strictly greater than the zero-value score, leaving the selected model empty after optimization.

To verify

• go test ./model/cf ./model/ctr
• git diff --check