[ty] implement `auto()` for `StrEnum` (#20524)
## Summary
see discussion here:
https://github.com/astral-sh/ty/issues/876#issuecomment-3310130167
https://docs.python.org/3/library/enum.html#enum.StrEnum
> Note Using
[auto](https://docs.python.org/3/library/enum.html#enum.auto) with
[StrEnum](https://docs.python.org/3/library/enum.html#enum.StrEnum)
results in the lower-cased member name as the value.
## Test Plan
- new mdtest
- also, added a test to assert the (already correct) behavior for
`IntEnum`
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Co-authored-by: David Peter <mail@david-peter.de>
