docs: state current behavior in MockComponent test comments

Rewrite process/history-narrating comments to describe what the code
does now, dropping issue references, the previous-crash narration, and
the "rather than a $: block" before/after framing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: thedavidmeister

packages/ui-components/src/__tests__/MockComponent.test.ts

@@ -36,10 +36,8 @@ test("mirrors the received props (excluding testid) into the shared store", () =
 });
 
 test("mounts more than one instance with distinct testids without crashing", () => {
-  // Reproduces the #1840 scenario: rendering more than one MockComponent in a
-  // single tree previously threw "RangeError: Invalid array length" from the
-  // reactive props store feedback loop. Distinct testids let each instance be
-  // queried individually.
+  // Rendering more than one MockComponent in a single tree is supported.
+  // Distinct testids let each instance be queried individually.
   render(MockComponentTest);
 
   const first = screen.getByTestId("first");

packages/ui-components/src/lib/__mocks__/MockComponent.svelte

@@ -9,9 +9,8 @@
 
 	// The received props are mirrored into a shared store so tests can assert on
 	// values (functions, objects) that cannot round-trip through DOM attributes.
-	// This runs in `afterUpdate` rather than a reactive `$:` block so it never
-	// re-enters the component's update cycle, which caused the invalid array
-	// length crash when multiple instances were mounted.
+	// This runs in `afterUpdate`, so the store write happens outside the
+	// component's update cycle and does not re-enter it.
 	afterUpdate(() => {
 		props.set($$restProps);
 	});