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.

examples/pdf-server/src/commands.ts

@@ -66,4 +66,5 @@ export type PdfCommand =
       getScreenshots: boolean;
     }
   | { type: "save_as"; requestId: string }
+  | { type: "get_viewer_state"; requestId: string }
   | { type: "file_changed"; mtimeMs: number };