Restore existing bindings when unbinding caught exceptions (#5256)

## Summary

In the latest release, we made some improvements to the semantic model,
but our modifications to exception-unbinding are causing some
false-positives. For example:

```py
try:
    v = 3
except ImportError as v:
    print(v)
else:
    print(v)
```

In the latest release, we started unbinding `v` after the `except`
handler. (We used to restore the existing binding, the `v = 3`, but this
was quite complicated.) Because we don't have full branch analysis, we
can't then know that `v` is still bound in the `else` branch.

The solution here modifies `resolve_read` to skip-lookup when hitting
unbound exceptions. So when store the "unbind" for `except ImportError
as v`, we save the binding that it shadowed `v = 3`, and skip to that.

Closes #5249.

Closes #5250.

crates/ruff_python_semantic/src/binding.rs

@@ -75,7 +75,7 @@ impl<'a> Binding<'a> {
     pub const fn is_unbound(&self) -> bool {
         matches!(
             self.kind,
-            BindingKind::Annotation | BindingKind::Deletion | BindingKind::UnboundException
+            BindingKind::Annotation | BindingKind::Deletion | BindingKind::UnboundException(_)
         )
     }
 
@@ -427,7 +427,11 @@ pub enum BindingKind<'a> {
     ///
     /// After the `except` block, `x` is unbound, despite the lack
     /// of an explicit `del` statement.
-    UnboundException,
+    ///
+    ///
+    /// Stores the ID of the binding that was shadowed in the enclosing
+    /// scope, if any.
+    UnboundException(Option<BindingId>),
 }
 
 bitflags! {