refactor(web): edit and wrap diagrams with the MCP server's code

- edit_diagram runs the MCP server's editDiagram: every new_xml is checked
  first, one cell per operation, and after the edit only the target page
  is checked, rejecting only errors this edit introduced. An unrelated
  problem elsewhere in the document no longer blocks every edit. The
  error lists each failed operation
- The streaming edit preview uses the MCP applyDiagramOperations
- Delete applyDiagramOperations (292 lines) and wrapWithMxFile from
  lib/utils.ts, and the unused hand-copied scripts/test-diagram-operations.mjs
- One blank document (BLANK_MXFILE) for the web app and the MCP preview,
  replacing four copies
- Saving a .drawio wraps a bare model with normalizeToMxfile
- The empty-diagram check uses hasCells, which also counts cells wrapped
  in a UserObject/object
- DiagramOperation is the MCP type
- The wrapped-cell and empty-diagram tests now run against the MCP code
- New e2e test: edit_diagram changes the canvas, and a failing edit
  leaves it as it was

app/api/chat/route.ts

@@ -23,7 +23,6 @@ import { findCachedResponse } from "@/lib/cached-responses"
 import {
     dropInvalidToolCalls,
     fixToolInputJson,
-    isMinimalDiagram,
     replaceHistoricalToolInputs,
     validateFileParts,
 } from "@/lib/chat-helpers"
@@ -50,6 +49,7 @@ import {
 import { allowPrivateUrls, isPrivateUrl } from "@/lib/ssrf-protection"
 import { getSystemPrompt } from "@/lib/system-prompts"
 import { getUserIdFromRequest } from "@/lib/user-id"
+import { hasCells } from "@/packages/mcp-server/src/pages.ts"
 
 // No explicit cap: a reasoning model can spend minutes planning before it emits
 // the tool call, so take whatever the host allows. Vercel's own default is 300s,
@@ -164,7 +164,7 @@ async function handleChatRequest(req: Request): Promise<Response> {
 
     // === CACHE CHECK START ===
     const isFirstMessage = messages.length === 1
-    const isEmptyDiagram = !xml || xml.trim() === "" || isMinimalDiagram(xml)
+    const isEmptyDiagram = !xml || !hasCells(xml)
 
     if (isFirstMessage && isEmptyDiagram) {
         const lastMessage = messages[0]