Fix option matching without cushion

[Danielxu0208]

Summary

• restore the missing separator between two Chinese answer patterns
• capture the option rather than its leading whitespace in the generic boundary pattern
• add regression coverage with cushion=False

Why

Two adjacent f-strings were implicitly concatenated, making the intended 只有选项 ... 是对 pattern unreachable. A later fallback pattern also captured the boundary as group 1, so matches such as reason A because were discarded when cushion matching was disabled.

Validation

• targeted checks for 只有选项 B 是对 and reason A because
• existing 答案是 B behavior retained
• git diff --check