fix(mcp): verifiable mutation results + refuse formatting edits in edit_page_text

edit_page_text reported "success" when asked to change formatting (e.g. remove
strikethrough): the markdown-strip fallback matched the bare text, the replace
preserved marks, and the tool returned success — so the agent believed it had
fixed something that never changed.

Two fixes, both in the shared @docmost/mcp DocmostClient so they reach BOTH the
standalone MCP server and the in-app AI chat (which loads @docmost/mcp):

- Verifiable result for every content mutator: mutatePageContent now computes a
  `verify` change-report (text inserted/deleted, blocks changed, per-mark-type
  delta, integrity/structure delta) via summarizeChange() and returns it on all
  mutators (incl. replaceImage via mutateLiveContentUnlocked). diffDocs is
  text-only, so the mark/structure delta is what surfaces formatting changes.
- edit_page_text hard-refuses formatting edits: applyTextEdits rejects an edit
  whose find/replace differ only in markdown markers (via stripBalancedWrappers,
  which strips balanced wrappers/links without trimming whitespace/emoji, so
  plain-text edits like trailing-space trims, snake_case, math are NOT refused).
  A fully-refused batch errors instead of silently succeeding.

Also updated the model-facing edit_page_text descriptions in BOTH tool layers
(packages/mcp/src/index.ts and ai-chat-tools.service.ts) to drop the misleading
"strip-and-retry tolerated" wording and point formatting changes to patch_node.

New unit tests: test/unit/diff-verify.test.mjs, test/unit/json-edit-refuse.test.mjs.

apps/server/src/core/ai-chat/tools/ai-chat-tools.service.ts

@@ -622,13 +622,19 @@ export class AiChatToolsService {
           '(so editing plain text next to a bold word keeps it bold, and ' +
           'editing inside a bold word keeps the new text bold). Each find must ' +
           'match exactly once unless replaceAll is set. The batch applies what ' +
-          'it can and returns applied[] + failed[]; a fully-unmatched batch ' +
-          'writes nothing and errors. find should be the literal rendered text ' +
-          '(no markdown). Markdown wrappers (**bold**, *italic*, `code`) and ' +
-          'trailing emoji are tolerated via a strip-and-retry fallback, but ' +
-          'plain text is preferred. Examples: edits:[{find:"teh",replace:"the"}]; ' +
-          'edits:[{find:"Hello world",replace:"Hello there"}] (crosses a bold ' +
-          'boundary). Reversible: the previous version is kept in page history.',
+          'it can and returns applied[] + failed[] plus a verify change-report ' +
+          '(the text/marks/structure that ACTUALLY changed — read it to confirm ' +
+          'your edit landed; do not assume success); a fully-unmatched batch ' +
+          'writes nothing and errors. find and replace are LITERAL text, not ' +
+          'markdown. This tool edits plain text ONLY and CANNOT add or remove ' +
+          'formatting marks: a formatting change — find/replace that differ only ' +
+          'in markdown markers (e.g. find:"~~x~~", replace:"x"), or a replace ' +
+          'containing **bold**/~~strike~~/`code` wrappers — is REFUSED into ' +
+          'failed[]. To change bold/italic/strike/code/link, read the block with ' +
+          'getPageJson and use patchNode (or updatePageJson) to set its marks. ' +
+          'Examples: edits:[{find:"teh",replace:"the"}]; edits:[{find:"Hello ' +
+          'world",replace:"Hello there"}] (crosses a bold boundary). Reversible: ' +
+          'the previous version is kept in page history.',
         inputSchema: z.object({
           pageId: z.string().describe('The id of the page to edit.'),
           edits: z