Converts between the stored format (text + PersonMention sidecar) and Tiptap
ProseMirror JSONContent. Round-trip invariant: serialize(deserialize(t,s)).text === t.
Handles multi-paragraph text (split/join on \n), sidecar deduplication, and
backward compat with old-format full-name sidecar entries.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>