[ty] Improve diagnostic range for non-subscriptable diagnostics (#21461)

## Summary

Currently our diagnostic only covers the range of the thing being
subscripted:

<img width="1702" height="312" alt="image"
src="https://github.com/user-attachments/assets/7e630431-e846-46ca-93c1-139f11aaba11"
/>

But it should probably cover the _whole_ subscript expression (arguably
the more "incorrect" bit is the `["foo"]` part of this expression, not
the `x` part of this expression!)

## Test Plan

Added a snapshot

Co-authored-by: Brent Westbrook
<36778786+ntBre@users.noreply.github.com>

crates/ty_python_semantic/resources/mdtest/snapshots/instance.md_-_Instance_subscript_-_`__getitem__`_unboun…_(b1b0f9ed2b7302b2).snap

@@ -0,0 +1,33 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: instance.md - Instance subscript - `__getitem__` unbound
+mdtest path: crates/ty_python_semantic/resources/mdtest/subscript/instance.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class NotSubscriptable: ...
+2 | 
+3 | a = NotSubscriptable()[0]  # error: [non-subscriptable]
+```
+
+# Diagnostics
+
+```
+error[non-subscriptable]: Cannot subscript object of type `NotSubscriptable` with no `__getitem__` method
+ --> src/mdtest_snippet.py:3:5
+  |
+1 | class NotSubscriptable: ...
+2 |
+3 | a = NotSubscriptable()[0]  # error: [non-subscriptable]
+  |     ^^^^^^^^^^^^^^^^^^^^^
+  |
+info: rule `non-subscriptable` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/subscript/instance.md

@@ -2,10 +2,12 @@
 
 ## `__getitem__` unbound
 
+<!-- snapshot-diagnostics -->
+
 ```py
 class NotSubscriptable: ...
 
-a = NotSubscriptable()[0]  # error: "Cannot subscript object of type `NotSubscriptable` with no `__getitem__` method"
+a = NotSubscriptable()[0]  # error: [non-subscriptable]
 ```
 
 ## `__getitem__` not callable

crates/ty_python_semantic/src/types/diagnostic.rs

@@ -2034,7 +2034,7 @@ pub(super) fn report_index_out_of_bounds(
 /// Emit a diagnostic declaring that a type does not support subscripting.
 pub(super) fn report_non_subscriptable(
     context: &InferContext,
-    node: AnyNodeRef,
+    node: &ast::ExprSubscript,
     non_subscriptable_ty: Type,
     method: &str,
 ) {

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

@@ -11369,11 +11369,11 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 .as_class_literal()
                 .is_some_and(|class| class.iter_mro(db, None).contains(&ClassBase::Generic))
             {
-                report_non_subscriptable(context, value_node.into(), value_ty, "__class_getitem__");
+                report_non_subscriptable(context, subscript, value_ty, "__class_getitem__");
             }
         } else {
             if expr_context != ExprContext::Store {
-                report_non_subscriptable(context, value_node.into(), value_ty, "__getitem__");
+                report_non_subscriptable(context, subscript, value_ty, "__getitem__");
             }
         }