* docs: list the summarize event tag and scope the extension example's claims
RUN_EVENT_STREAM.md listed seven middleware tags, but the catalog and the
run event stream contract carry eight; `summarize` is emitted by
SummarizationMiddleware for every successful summary result. Add the tag,
describe the event, and pin the documented list to MIDDLEWARE_EVENT_TAGS so
the next tag cannot drift silently. summarization.md now lists all eight
`changes` fields of that event.
The extension example said it showed all five (README, pyproject, module
docstrings, test name) or every contribution kind, while ExtensionRegistry
has eight. Say it demonstrates five, name the three it does not, point to
the observers guide and the bookmarks plugin, and assert in the example's
test that the two observer kinds stay unregistered.
* docs(changelog): reference #6582
* docs: clarify extension example package layout scope
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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