…ith Python type checkers (Pyright/MyPy).

Rationale: The current type definition strictly requires dict[str, int]. Because standard dictionaries are invariant, this definition rejects dictionaries keyed by StrEnum, despite StrEnum being a subclass of str.

This forces downstream users to cast their data types or lose type safety. The example below illustrates the false positive error generated by the current signature:

```Python
import enum

class MyEnum(enum.StrEnum):
  # Minimal replacement of e.g. research.shared.configs.base.StrEnumEnum
  a = enum.auto()
  b = enum.auto()

def func_we_control() -> dict[MyEnum, int]:
  # It's important to keep the return as MyEnum, so that other code knows the keys of this.
  return {MyEnum.a: 1}

def func_we_dont_control(x: dict[str, int]) -> None:
  del x
  pass

func_we_dont_control(func_we_control())
```

Pyright Error:

Type parameter "_KT@dict" is invariant, but "MyEnum" is not the same as "str"

PiperOrigin-RevId: 855783481