fix(mcp-server): keep both pages when get_diagram meets a page export, and more review fixes

Found by the PR review, each with a test that failed first:
- get_diagram during a page export returned the one-page projection on
  screen as the whole document (6 of 6 times when timed so). The preview
  page no longer answers a sync while a projection shows, and syncs after
  reloading, so the poll that restores the real document exports it.
- Exports are numbered on the server too: a late result of an export that
  timed out was saved as the next export's file.
- In Chrome, a new_xml with a syntax error counted the <parsererror>
  element as a second cell, so the web app rejected edits that auto-fix
  repairs ("must contain exactly one cell").
- hasCells missed single-quoted ids, so screenshot_diagram called such a
  diagram empty and auto-save never created its file.
- A literal \n directly under a <diagram> that has a model passed
  validation; only text-only pages are compressed data.
- A wrapped mxCell repeating its UserObject's id took the wrapper's place
  in edits, so delete and update left an empty or nested wrapper.
- Bare cells with a shape or edge id of "0" or "1" are rejected with a
  clear message instead of being renamed, which broke their edges.
- DRAWIO_DATA_DIR expands ~, which JSON configs pass on as it is.

tests/e2e/diagram-content.spec.ts

@@ -135,6 +135,36 @@ test("display_diagram replaces the document with the fixed diagram", async ({
     await expect(canvas.getByText("Second", { exact: true })).toHaveCount(0)
 })
 
+test("an edit with a fixable cell is fixed, not rejected", async ({
+    page: p,
+}) => {
+    // Chrome's DOMParser puts a <parsererror> next to the cell, which used
+    // to count as a second cell
+    const canvas = await mockReplies(p, [
+        streamedToolCall("display_diagram", { xml: cell("a", "Alpha", 40) }),
+        streamedToolCall("edit_diagram", {
+            operations: [
+                {
+                    operation: "add",
+                    cell_id: "c",
+                    new_xml: cell("c", "Gamma", 400).replace(
+                        "</mxCell>",
+                        "</mxcell>",
+                    ),
+                },
+            ],
+        }),
+    ])
+    await sendMessage(p, "Draw a box")
+    await waitForCompleteCount(p, 1)
+    await sendMessage(p, "Add another box")
+    await waitForCompleteCount(p, 2)
+    await expect(canvas.getByText("Gamma", { exact: true })).toBeVisible({
+        timeout: 15000,
+    })
+    await expect(p.getByText(/exactly one cell/)).toHaveCount(0)
+})
+
 test("edit_diagram applies all operations or none", async ({ page: p }) => {
     const canvas = await mockReplies(p, [
         streamedToolCall("display_diagram", {