[ty] Improve diagnostics when NotImplemented is called (#21523)

## Summary

Fixes https://github.com/astral-sh/ty/issues/1571.

I realised I was overcomplicating things when I described what we should
do in that issue description. The simplest thing to do here is just to
special-case call expressions and short-circuit the call-binding
machinery entirely if we see it's `NotImplemented` being called. It
doesn't really matter if the subdiagnostic doesn't fire when a union is
called and one element of the union is `NotImplemented` -- the
subdiagnostic doesn't need to be exhaustive; it's just to help people in
some common cases.

## Test Plan

Added snapshots

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -7912,6 +7912,24 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 ty
             });
 
+        if callable_type.is_notimplemented(self.db()) {
+            if let Some(builder) = self
+                .context
+                .report_lint(&CALL_NON_CALLABLE, call_expression)
+            {
+                let mut diagnostic = builder.into_diagnostic("`NotImplemented` is not callable");
+                diagnostic.annotate(
+                    self.context
+                        .secondary(&**func)
+                        .message("Did you mean `NotImplementedError`?"),
+                );
+                diagnostic.set_concise_message(
+                    "`NotImplemented` is not callable - did you mean `NotImplementedError`?",
+                );
+            }
+            return Type::unknown();
+        }
+
         // Special handling for `TypedDict` method calls
         if let ast::Expr::Attribute(ast::ExprAttribute { value, attr, .. }) = func.as_ref() {
             let value_type = self.expression_type(value);