Fix incorrect type inference for yield from with iterators

[dhanusharer]

Fixes #17449

Previously, when yield from was used with an Iterator[T], mypy
failed to correctly propagate the yielded item type and inferred Any.
This allowed incompatible generator return types to pass type checking.

This change extracts the iterator element type directly in
visit_yield_from_expr, enabling proper type comparison.

A regression test has been added.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

bandersnatch (https://github.com/pypa/bandersnatch)
+ src/bandersnatch_storage_plugins/filesystem.py: note: In member "scandir" of class "FilesystemStorage":
+ src/bandersnatch_storage_plugins/filesystem.py:213: error: Incompatible types in "yield from" (actual type "str", expected type "DirEntry[Any]")  [misc]

mypy (https://github.com/python/mypy)
+ mypy/report.py:150: error: Incompatible types in "yield from" (actual type "str", expected type "tuple[int, str]")  [misc]