[ty] Fix attribute writes to unions/intersections including modules (#18313)

## Summary

Fix a bug that involved writes to attributes on union/intersection types
that included modules as elements.

This is a prerequisite to avoid some ecosystem false positives in
https://github.com/astral-sh/ruff/pull/18312

## Test Plan

Added regression test

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -1698,6 +1698,36 @@ reveal_type(outer.nested.inner.Outer.Nested.Inner.attr)  # revealed: int
 outer.nested.inner.Outer.Nested.Inner.attr = "a"
 ```
 
+### Unions of module attributes
+
+`mod1.py`:
+
+```py
+global_symbol: str = "a"
+```
+
+`mod2.py`:
+
+```py
+global_symbol: str = "a"
+```
+
+```py
+import mod1
+import mod2
+
+def _(flag: bool):
+    if flag:
+        mod = mod1
+    else:
+        mod = mod2
+
+    mod.global_symbol = "b"
+
+    # error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to attribute `global_symbol` on type `<module 'mod1'> | <module 'mod2'>`"
+    mod.global_symbol = 1
+```
+
 ## Literal types
 
 ### Function-literal attributes