[ty] Narrowing for hasattr() (#18053)

crates/ty_python_semantic/resources/mdtest/narrow/hasattr.md

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+# Narrowing using `hasattr()`
+
+The builtin function `hasattr()` can be used to narrow nominal and structural types. This is
+accomplished using an intersection with a synthesized protocol:
+
+```py
+from typing import final
+
+class Foo: ...
+
+@final
+class Bar: ...
+
+def f(x: Foo):
+    if hasattr(x, "spam"):
+        reveal_type(x)  # revealed: Foo & <Protocol with members 'spam'>
+        reveal_type(x.spam)  # revealed: object
+
+    if hasattr(x, "not-an-identifier"):
+        reveal_type(x)  # revealed: Foo
+
+def y(x: Bar):
+    if hasattr(x, "spam"):
+        reveal_type(x)  # revealed: Never
+        reveal_type(x.spam)  # revealed: Never
+```