test(NotAvailablePanel, AttachmentViews): add 19-case vitest suites

Add vitest coverage for two remaining chat/FilesTab components:
- NotAvailablePanel: 5 cases — heading, monospace runtime name, helper
  text, SVG aria-hidden, different runtime display
- AttachmentViews (PendingAttachmentPill + AttachmentChip): 14 cases —
  file name/size rendering, formatSize units, remove/download callbacks,
  aria-labels, tone styles, SVG glyph

Fix: use Object.defineProperty to override jsdom File size (jsdom
ignores the size constructor arg); use afterEach(cleanup) to prevent
accumulated DOM elements between NotAvailablePanel tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/tabs/FilesTab/__tests__/NotAvailablePanel.test.tsx

@@ -0,0 +1,49 @@
+// @vitest-environment jsdom
+/**
+ * Tests for NotAvailablePanel — full-tab placeholder for unsupported runtimes.
+ *
+ * Coverage:
+ *   - Renders heading "Files not available"
+ *   - Renders runtime name in monospace span
+ *   - Renders helper text referencing Chat tab
+ *   - SVG icon is aria-hidden
+ *   - Different runtime names display correctly
+ */
+import React from "react";
+import { render, screen, cleanup } from "@testing-library/react";
+import { afterEach, describe, expect, it } from "vitest";
+import { NotAvailablePanel } from "../NotAvailablePanel";
+
+afterEach(cleanup);
+
+describe("NotAvailablePanel", () => {
+  it("renders heading 'Files not available'", () => {
+    render(<NotAvailablePanel runtime="external" />);
+    expect(screen.getByText("Files not available")).toBeTruthy();
+  });
+
+  it("renders the runtime name in monospace", () => {
+    render(<NotAvailablePanel runtime="external" />);
+    expect(screen.getByText("external")).toBeTruthy();
+    const runtimeSpan = screen.getByText("external");
+    expect(runtimeSpan.tagName.toLowerCase()).toBe("span");
+  });
+
+  it("renders helper text referencing Chat tab", () => {
+    render(<NotAvailablePanel runtime="external" />);
+    expect(screen.getByText(/chat tab/i)).toBeTruthy();
+  });
+
+  it("renders SVG icon as aria-hidden", () => {
+    render(<NotAvailablePanel runtime="external" />);
+    const svg = document.querySelector("svg");
+    expect(svg?.getAttribute("aria-hidden")).toBe("true");
+  });
+
+  it("displays different runtime names correctly", () => {
+    render(<NotAvailablePanel runtime="hermes" />);
+    expect(screen.getByText("hermes")).toBeTruthy();
+    // "runtime" appears in the text node after the hermes span
+    expect(screen.getByText(/runtime, whose filesystem/i)).toBeTruthy();
+  });
+});