Explicitly require Y/N for Command._ask

Is your feature request related to a problem? Please describe.
Had pymanager uninstall <tag> in my terminal and in a dumb panic on my split keyboard, submitted the characters yhn:

pymanager uninstall 3.13.11
Uninstall Python 3.13.11? [Y/n] yhn

Expected to cancel/break since yhn does not match a provided option - y or n.
In actuality, it proceeded to remove that python installation.

Describe the solution you'd like
Literally match for the submitted character y or n.

Describe alternatives you've considered
Not panicking and hitting n (or hitting anything else really, as long as it does not start with y)

Additional context
Was a silly mistake that costed me enough rebuild time that I came back just to figure out why it happened.
https://github.com/python/pymanager/blob/main/src/manage/commands.py#L624

https://github.com/python/pymanager/blob/682d0f87a0eadb3e137b065cda0b3a74d8e83fa2/src/manage/commands.py#L610-L624

Since we match via startswith, it does not matter what the remaining characters are - as long as expect_char is the first char, then _ask evaluates.

从 src/manage/commands.py 的第 610-624 行附近开始，重点查看入口点 Command._ask 和相关的 prompt 逻辑。验证诸如 yhn 这样的输入会被拒绝，而不是被当作 y 处理，同时预期的 Y/N 响应仍保持其预期行为。