fix(chat): the AI reads and draws the page the user is viewing

The model always got the first page and display_diagram replaced the whole
file with one page, so drawing on page 3 of a multi-page file deleted the
other pages (#241), and edits went to page 1 while the user looked at
another page.

Now a turn records the page the user is viewing when the message is sent.
That page's model goes to the model (also as previousXml, on regenerate and
in edit_diagram error messages), selected shapes are sent on any page,
display_diagram and append_diagram replace only that page (a drawing that
brings several pages still replaces the file), edit_diagram targets it,
and the streaming preview draws on it. Compact cells and default styles
take the layer and the edges of that page, in the preview and in
editDiagram. The editor bridge replaces the current page in place when the
other pages are unchanged, so Ctrl+Z undoes an AI change on a multi-page
file in one step; when a full load is needed, the user's page is shown
again afterwards. Version summaries and the compare dialog count the page
the change was made on.

tests/e2e/workspace.spec.ts

@@ -1001,7 +1001,7 @@ test.describe("Workspace", () => {
         await expect(canvas.getByText("Old A")).toBeVisible()
         await sendMessage(page, "Create a flowchart")
         await waitForComplete(page)
-        // Multi-page documents are loaded in full: the card is the way back
+        // The card is one way back
         await page.waitForTimeout(2000)
         await page.locator('[data-testid="version-undo"]').click()
         await expect(canvas.getByText("Old A")).toBeVisible()
@@ -1533,7 +1533,7 @@ test.describe("Workspace", () => {
         await expect.poll(savedPageId, { timeout: 10000 }).toBe("orig")
     })
 
-    test("Ctrl+Z after an AI drawing gives the page its id back", async ({
+    test("an AI drawing keeps the page's id, for links to the page", async ({
         page,
     }) => {
         await page.route("**/api/chat", drawThenEdit())
@@ -1567,7 +1567,7 @@ test.describe("Workspace", () => {
                     .querySelector("diagram")
                     ?.getAttribute("id")
             })
-        await expect.poll(savedPageId, { timeout: 10000 }).toBe("page-1")
+        await expect.poll(savedPageId, { timeout: 10000 }).toBe("orig")
         await canvas
             .locator(".geDiagramContainer")
             .click({ position: { x: 10, y: 10 } })
@@ -1576,7 +1576,62 @@ test.describe("Workspace", () => {
         await expect.poll(savedPageId, { timeout: 10000 }).toBe("orig")
         await page.keyboard.press("ControlOrMeta+Shift+z")
         await expect(canvas.getByText("Process", { exact: true })).toBeVisible()
-        await expect.poll(savedPageId, { timeout: 10000 }).toBe("page-1")
+        await expect.poll(savedPageId, { timeout: 10000 }).toBe("orig")
+    })
+
+    test("the AI reads and draws the page the user is viewing", async ({
+        page,
+    }) => {
+        const bodies: any[] = []
+        await page.route(
+            "**/api/chat",
+            drawThenEdit((body) => bodies.push(body)),
+        )
+        await openApp(page)
+        await openDrawioFile(page, "two.drawio", TWO_PAGE_FILE)
+        const canvas = getIframeContent(page)
+        await expect(canvas.getByText("Old A")).toBeVisible()
+        // The second page, through draw.io's own tabs
+        await drawioTabs(page).getByText("P1", { exact: true }).click()
+        await expect(canvas.getByText("Old B")).toBeVisible()
+        await sendMessage(page, "Create a flowchart")
+        await waitForComplete(page)
+        // The model got the page on screen
+        expect(bodies[0].xml).toContain("Old B")
+        expect(bodies[0].xml).not.toContain("Old A")
+        // Its drawing replaced that page, and only that page
+        await expect(canvas.getByText("Process", { exact: true })).toBeVisible()
+        await expect(canvas.getByText("Old B")).toHaveCount(0)
+        const pagesIn = (xml: string) =>
+            Object.fromEntries(
+                Array.from(
+                    xml.matchAll(
+                        /<diagram[^>]*\bid="([^"]+)"[^>]*>([\s\S]*?)<\/diagram>/g,
+                    ),
+                    (m) => [m[1], m[2]],
+                ),
+            )
+        await expect
+            .poll(
+                async () => {
+                    const pages = pagesIn(await savedDiagram(page))
+                    return {
+                        p0: pages.p0?.includes("Old A"),
+                        p1: pages.p1?.includes("Process"),
+                        oldB: pages.p1?.includes("Old B"),
+                    }
+                },
+                { timeout: 10000 },
+            )
+            .toEqual({ p0: true, p1: true, oldB: false })
+        // One undo step takes the drawing back, as on a one-page file
+        await canvas
+            .locator(".geDiagramContainer")
+            .click({ position: { x: 10, y: 10 } })
+        await page.keyboard.press("ControlOrMeta+z")
+        await expect(canvas.getByText("Old B")).toBeVisible()
+        await drawioTabs(page).getByText("P0", { exact: true }).click()
+        await expect(canvas.getByText("Old A")).toBeVisible()
     })
 
     test("page settings the AI writes in single quotes are applied too", async ({