[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`

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/ty_python_semantic/resources/mdtest/enums.md

@@ -267,6 +267,11 @@ reveal_type(Color.red)
 
 ### Using `auto()`
 
+```toml
+[environment]
+python-version = "3.11"
+```
+
 ```py
 from enum import Enum, auto
 from ty_extensions import enum_members
@@ -277,6 +282,50 @@ class Answer(Enum):
 
 # revealed: tuple[Literal["YES"], Literal["NO"]]
 reveal_type(enum_members(Answer))
+
+reveal_type(Answer.YES.value)  # revealed: Literal[1]
+reveal_type(Answer.NO.value)  # revealed: Literal[2]
+```
+
+Usages of `auto()` can be combined with manual value assignments:
+
+```py
+class Mixed(Enum):
+    MANUAL_1 = -1
+    AUTO_1 = auto()
+    MANUAL_2 = -2
+    AUTO_2 = auto()
+
+reveal_type(Mixed.MANUAL_1.value)  # revealed: Literal[-1]
+reveal_type(Mixed.AUTO_1.value)  # revealed: Literal[1]
+reveal_type(Mixed.MANUAL_2.value)  # revealed: Literal[-2]
+reveal_type(Mixed.AUTO_2.value)  # revealed: Literal[2]
+```
+
+When using `auto()` with `StrEnum`, the value is the lowercase name of the member:
+
+```py
+from enum import StrEnum, auto
+
+class Answer(StrEnum):
+    YES = auto()
+    NO = auto()
+
+reveal_type(Answer.YES.value)  # revealed: Literal["yes"]
+reveal_type(Answer.NO.value)  # revealed: Literal["no"]
+```
+
+Using `auto()` with `IntEnum` also works as expected:
+
+```py
+from enum import IntEnum, auto
+
+class Answer(IntEnum):
+    YES = auto()
+    NO = auto()
+
+reveal_type(Answer.YES.value)  # revealed: Literal[1]
+reveal_type(Answer.NO.value)  # revealed: Literal[2]
 ```
 
 Combining aliases with `auto()`: