[red-knot] Fix: Infer type for typing.Union[..] tuple expression (#14510)

## Summary

Fixes a panic related to sub-expressions of `typing.Union` where we fail
to store a type for the `int, str` tuple-expression in code like this:
```
x: Union[int, str] = 1
```

relates to [my
comment](https://github.com/astral-sh/ruff/pull/14499#discussion_r1851794467)
on #14499.

## Test Plan

New corpus test

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4568,10 +4568,14 @@ impl<'db> TypeInferenceBuilder<'db> {
                 UnionType::from_elements(self.db, [param_type, Type::none(self.db)])
             }
             KnownInstanceType::Union => match parameters {
-                ast::Expr::Tuple(t) => UnionType::from_elements(
-                    self.db,
-                    t.iter().map(|elt| self.infer_type_expression(elt)),
-                ),
+                ast::Expr::Tuple(t) => {
+                    let union_ty = UnionType::from_elements(
+                        self.db,
+                        t.iter().map(|elt| self.infer_type_expression(elt)),
+                    );
+                    self.store_expression_type(parameters, union_ty);
+                    union_ty
+                }
                 _ => self.infer_type_expression(parameters),
             },
             KnownInstanceType::TypeVar(_) => todo_type!(),

crates/red_knot_workspace/resources/test/corpus/95_annotation_union.py

@@ -0,0 +1,3 @@
+from typing import Union
+
+x: Union[int, str] = 1