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.

Start at src/manage/commands.py around lines 610-624, especially the Command._ask entry point and the linked prompt logic. Verify that an input such as yhn is rejected rather than treated as y, while the intended Y/N responses retain their expected behavior.