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

tests/unit/style-classes.test.ts

@@ -9,10 +9,15 @@ import {
 import {
     addDefaultStyles,
     applyStyleClasses,
-    expandStyles,
     readStyleClasses,
 } from "@/packages/mcp-server/src/style-classes.ts"
 
+/** The named styles of a model's XML expanded, with the default styles */
+const expandStyles = (xml: string) => {
+    const { classes, xml: cells } = readStyleClasses(xml)
+    return addDefaultStyles(applyStyleClasses(cells, classes))
+}
+
 const geometry =
     '<mxGeometry x="0" y="0" width="80" height="40" as="geometry"/>'
 const cell = (id: string, style: string, extra = "") =>