Keep MCP updates of stored links working and drop unsafe pasted links

- MCP update_document no longer applies the new-document URL check; only
  create does. Updates go through edit.replace, which judges against the
  stored document.
- The introduced-URL check also refuses surrounding whitespace and a
  scheme hidden by whitespace, and reports the structural path.
- A new local link (paste, import) whose address fails checkUrl is
  unwrapped to its text before it syncs, so a paste cannot send a batch
  the server refuses.
- Tests: MCP update of a stored backslash link, moving and duplicating a
  stored URL, a hidden character through room.apply, Service.initial
  refusals, the paste guard, and a paste e2e.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>