feat(pdf-server): get_viewer_state interact action (#590)

* feat(pdf-server): get_viewer_state interact action

New interact action that returns a JSON snapshot of the live viewer:
{currentPage, pageCount, zoom, displayMode, selectedAnnotationIds,
 selection: {text, contextBefore, contextAfter, boundingRect} | null}.

The viewer already pushes selection passively via setModelContext as
<pdf-selection> tags, but not all hosts surface model-context. This gives
the model an explicit pull.

selection.boundingRect is a single bbox in PDF points (top-left origin,
y-down) so it can be fed straight back into add_annotations. selection is
null when nothing is selected or the selection is outside the text-layer.

Wiring: new PdfCommand variant -> processCommands case ->
handleGetViewerState -> submit_viewer_state (new app-only tool, mirrors
submit_save_data) -> waitForViewerState -> text content block.

Also fills a gap in the display_pdf description: it listed interact
actions but was missing save_as; added that and get_viewer_state.

e2e: two tests covering selection:null and a programmatic text-layer
selection.

* test(pdf-server): fix get_viewer_state e2e race + assertions

readLastToolResult clicked .last() before the interact result panel
existed (callInteract doesn't block), so it expanded the display_pdf
panel instead. Wait for the expected panel count first.

Also: basic-host renders the full CallToolResult JSON, with the state
double-escaped inside content[0].text. Parse instead of regex-matching.

playwright.config.ts: honor PW_CHANNEL env to use system Chrome locally
when the bundled chromium_headless_shell is broken.

tests/e2e/pdf-annotations.spec.ts

@@ -314,3 +314,93 @@ test.describe("PDF Server - Annotations", () => {
     );
   });
 });
+
+/**
+ * Read the most recent interact result text from the basic-host UI.
+ * Waits for the result-panel count to reach `expectedCount` first —
+ * `callInteract` doesn't block, so `.last()` would otherwise race to the
+ * previous (display_pdf) panel.
+ */
+async function readLastToolResult(
+  page: Page,
+  expectedCount: number,
+): Promise<string> {
+  const panels = page.locator('text="📤 Tool Result"');
+  await expect(panels).toHaveCount(expectedCount, { timeout: 30000 });
+  await panels.last().click();
+  const pre = page.locator("pre").last();
+  await expect(pre).toBeVisible({ timeout: 5000 });
+  return (await pre.textContent()) ?? "";
+}
+
+/** Unwrap basic-host's `CallToolResult` JSON to the first text block. */
+function unwrapTextResult(raw: string): string {
+  const parsed = JSON.parse(raw) as {
+    content?: { type: string; text?: string }[];
+  };
+  const block = parsed.content?.find((c) => c.type === "text");
+  if (!block?.text) throw new Error(`No text block in: ${raw.slice(0, 200)}`);
+  return block.text;
+}
+
+test.describe("PDF Server - get_viewer_state", () => {
+  test("returns page/zoom/mode and selection:null when nothing is selected", async ({
+    page,
+  }) => {
+    await loadPdfServer(page);
+    await waitForPdfCanvas(page);
+
+    const viewUUID = await extractViewUUID(page);
+
+    await callInteract(page, { viewUUID, action: "get_viewer_state" });
+    const raw = await readLastToolResult(page, 2);
+    const state = JSON.parse(unwrapTextResult(raw));
+
+    expect(state.currentPage).toBe(1);
+    expect(state.pageCount).toBeGreaterThan(1);
+    expect(typeof state.zoom).toBe("number");
+    expect(state.displayMode).toBe("inline");
+    expect(state.selection).toBeNull();
+    expect(Array.isArray(state.selectedAnnotationIds)).toBe(true);
+  });
+
+  test("returns selected text and bounding rect when text-layer text is selected", async ({
+    page,
+  }) => {
+    await loadPdfServer(page);
+    await waitForPdfCanvas(page);
+
+    const viewUUID = await extractViewUUID(page);
+    const app = getAppFrame(page);
+
+    // Programmatically select the contents of the first text-layer span.
+    const selectedText = await app
+      .locator("#text-layer span")
+      .first()
+      .evaluate((span) => {
+        const range = span.ownerDocument.createRange();
+        range.selectNodeContents(span);
+        const sel = span.ownerDocument.defaultView!.getSelection()!;
+        sel.removeAllRanges();
+        sel.addRange(range);
+        return sel.toString().replace(/\s+/g, " ").trim();
+      });
+    expect(selectedText.length).toBeGreaterThan(0);
+
+    await callInteract(page, { viewUUID, action: "get_viewer_state" });
+    const raw = await readLastToolResult(page, 2);
+    const state = JSON.parse(unwrapTextResult(raw));
+
+    expect(state.currentPage).toBe(1);
+    expect(state.selection).not.toBeNull();
+    expect(state.selection.text).toContain(selectedText);
+    expect(state.selection.boundingRect).toEqual(
+      expect.objectContaining({
+        x: expect.any(Number),
+        y: expect.any(Number),
+        width: expect.any(Number),
+        height: expect.any(Number),
+      }),
+    );
+  });
+});