[ty] Avoid double-inference on non-tuple argument to `Annotated` (#21837)

## Summary

If you pass a non-tuple to `Annotated`, we end up running inference on
it twice. I _think_ the only case here is `Annotated[]`, where we insert
a (fake) empty `Name` node in the slice.

Closes https://github.com/astral-sh/ty/issues/1801.

crates/ty_python_semantic/resources/mdtest/invalid_syntax.md

@@ -128,3 +128,16 @@ InvalidEmptyUnion = Union[]
 def _(u: InvalidEmptyUnion):
     reveal_type(u)  # revealed: Unknown
 ```
+
+### `typing.Annotated`
+
+```py
+from typing import Annotated
+
+# error: [invalid-syntax] "Expected index or slice expression"
+# error: [invalid-type-form] "Special form `typing.Annotated` expected at least 2 arguments (one type and at least one metadata element)"
+InvalidEmptyAnnotated = Annotated[]
+
+def _(a: InvalidEmptyAnnotated):
+    reveal_type(a)  # revealed: Unknown
+```

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

@@ -1172,7 +1172,11 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     )
                     .in_type_expression(db, self.scope(), None)
                     .unwrap_or_else(|err| err.into_fallback_type(&self.context, subscript, true));
-                self.store_expression_type(arguments_slice, ty);
+                // Only store on the tuple slice; non-tuple cases are handled by
+                // `infer_subscript_load_impl` via `infer_expression`.
+                if arguments_slice.is_tuple_expr() {
+                    self.store_expression_type(arguments_slice, ty);
+                }
                 ty
             }
             SpecialFormType::Literal => match self.infer_literal_parameter_type(arguments_slice) {