`yield from` combines single-type iterators

**Describe the bug**
When using `yield from` on a variable of type `Iterable[a] | Iterable[b]`, the return type seems to combine the two single-type iterator possibilities into a generator `Generator[a | b, ...]`.
This causes a very basic direct wrapper as in the sample code below to fail type checking.

**Code or Screenshots**

```python
from typing import Iterable

def single_type_iterable() -> Iterable[int] | Iterable[str]:
raise NotImplementedError()

def wrapper() -> Iterable[int] | Iterable[str]:
yield from single_type_iterable() # ERROR
```
```
iter_demo.py:7:16 - error: Return type of generator function must be compatible with "Generator[int | str, Any, Any]"
  Type "Generator[int | str, Unknown, Unknown]" is not assignable to type "Iterable[int] | Iterable[str]"
    "Generator[int | str, Unknown, Unknown]" is not assignable to "Iterable[int]"
      Type parameter "_T_co@Iterable" is covariant, but "int | str" is not a subtype of "int"
        Type "int | str" is not assignable to type "int"
          "str" is not assignable to "int"
    "Generator[int | str, Unknown, Unknown]" is not assignable to "Iterable[str]"
      Type parameter "_T_co@Iterable" is covariant, but "int | str" is not a subtype of "str"
        Type "int | str" is not assignable to type "str"
... (reportReturnType)
1 error, 0 warnings, 0 informations
```

**VS Code extension or command-line**
Verified to reproduce in CLI v1.1.407 via npx/npm (plain call, no config provided)

Start by running the minimal example in iter_demo.py with the Pyright CLI and inspect how yield from handles the Iterable[int] | Iterable[str] return type. The work is done when this wrapper type-checks without the reported error and the behavior is covered by a regression test.

Written by the indexing model from the issue text.