refactor(mcp-server): remove dead code and fold repeated tool plumbing

- index.ts: registerTool() does the session check, try/catch and the
  write queue for all 15 tools; fail()/ok(), syncFromBrowser(),
  commitDocument() and describePages() replace the copied blocks; drop
  pickPageSelector, currentSession.version, the input ?? {} defaults;
  add_page reuses prepareNewModel() split out of prepareNewDiagram
- xml-validation.ts: drop the hand-written syntax checks that saxes
  already reports, MAX_XML_SIZE and the stale header
- http-server.ts: one pendingExport object, no unread response fields,
  no CORS/OPTIONS branch for same-origin requests, fewer exports
- diagram-operations parseNewCell(), addHistory returns void,
  normalizeToMxfile without host, foldCells without layer, expandStyles
  removed, unused preview.css rule removed, no .d.ts output
- xml-examples.ts styleRules() shares the Styles paragraphs with the
  web prompt
- tests: setupFiles installs the DOM polyfill once, server-wiring uses
  the SDK client, mcp-core.test.ts drops the cases MCP tests already
  cover

packages/mcp-server/tests/xml-reference.test.ts

@@ -4,13 +4,7 @@
  * whether draw.io renders it as intended is checked by hand.
  */
 
-import { beforeAll, describe, expect, it } from "vitest"
-import { installDomPolyfill } from "../src/dom.ts"
-
-beforeAll(() => {
-    installDomPolyfill()
-})
-
+import { describe, expect, it } from "vitest"
 import { prepareNewDiagram } from "../src/new-diagram.ts"
 import { hasCells } from "../src/pages.ts"
 import { XML_REFERENCE } from "../src/xml-reference.ts"