test(word-addin): e2e proof of the markdown context and marker-quote fallbacks

WHY A MOCK EXTENSION
The Office mock's body exposed only text/search - no paragraphs or
tables - so the structured read was untestable (it would throw and fall
back to flat text in every spec). The seed gains documentBlocks: when
present, body.paragraphs/body.tables describe those blocks; when absent
(every pre-existing spec) the collections stay undefined, which is
exactly the shape of a host without the structure APIs, so the flat-text
fallback keeps its coverage for free.

WHAT THE THREE TESTS PIN DOWN (each fails on flat body.text)
1. document_context carries "# " headings, list labels with the real
   Word listString, and pipe tables - flat text contains none of these.
2. An edit whose <original> is "# Definitions" (the renderer's heading
   marker included) still lands as ONE tracked change on "Definitions":
   the verbatim search misses, the stripped-marker retry locates the
   underlying text. On flat-text code this edit dies as
   "Skipped - source text was not found."
3. A citation chip quoting "a. Affiliate means..." (list label included)
   still selects the real passage in Word via the same stripped retry;
   before, both the exact and case-insensitive searches miss and nothing
   is ever selected.

Full suite: 266 passed (chromium + webkit), 6 of them new.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>