Search never returns yzhang.markdown-all-in-one for any "markdown" query

## Summary

`yzhang.markdown-all-in-one` ("Markdown All in One") is published on open-vsx.org
and resolvable directly by namespace/name, but the search API never returns it
for any query related to "markdown" — even though it has far more downloads
than everything that *does* get returned.

## Steps to reproduce

Direct lookup succeeds:

curl -s https://open-vsx.org/api/yzhang/markdown-all-in-one

returns valid metadata: version 3.6.2, published 2024-01-16T04:14:26Z,
637,756 downloads.

Search does not return it:

curl -s "https://open-vsx.org/api/-/search?query=markdown&size=20"
curl -s "https://open-vsx.org/api/-/search?query=markdown+all+in+one&size=10"

Neither call includes `yzhang.markdown-all-in-one` anywhere in the results.
The top 20 results for `query=markdown` are all extensions with a small
fraction of its download count — none of them is even close to as popular.

## Impact

This isn't just a ranking quirk — the extension is fully absent from the
result set. Any client that installs via the search/browse flow (e.g. the
Extensions view in VS Code-compatible IDEs like Theia) cannot find or
install it under any phrasing of its name.

## Suspected root cause

Possibly related to #12491 / #12466 (disabling `elasticsearchClearOnStart`
in prod/staging, merged 2026-08-13). Per that PR's description, before the
fix, concurrent pod starts could race the index rebuild and serve a
half-populated index, and index freshness after a bad rebuild only comes
from "the daily JobRunr soft-update and the incremental writes on publish"
— neither of which re-adds a document that was dropped by an earlier bad
rebuild if the extension hasn't been republished since. `markdown-all-in-one`
was last published 2024-01-16, well before that fix, which fits the profile
of an entry that got dropped from the index at some point and never
self-healed.

If that's right, a one-off `POST /admin/update-search-index` full rebuild
should fix this case (and likely any other extension silently missing from
search for the same reason) — but it'd be worth checking why the daily
soft-update doesn't recover entries like this rather than only patching it
once.

## Environment

Reproduced 2026-09-04 via direct `curl` against the public open-vsx.org API,
independent of any specific client.

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Assisted-by: Anthropic Claude Sonnet 5 (claude-sonnet-5) — used to investigate and draft this report; findings verified against the live open-vsx.org API.