Add test for exhaustiveness checking

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

diff --git a/crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md b/crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md
index d8e23dd..c48517e 100644
--- a/crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md
+++ b/crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md
@@ -937,6 +937,20 @@
         reveal_type(xs)  # revealed: (OpenItem & ~list[Unknown]) | (list[OpenItem] & ~list[Unknown])
 ```
 
+#### Exhaustiveness checking
+
+In relaxed mode, exhaustiveness checking is harder to achieve:
+
+```py
+# TODO
+# error: [invalid-return-type] "Function can implicitly return `None`, which is not assignable to return type `str`"
+def _(xs: list[str] | set[str]) -> str:
+    if isinstance(xs, list):
+        return "it's a list!"
+    elif isinstance(xs, set):
+        return "it's a set!"
+```
+
 ## Narrowing generic defaults in Python 3.13
 
 When a type parameter has a bare `Any` default, narrowing still materializes the substituted