Align the 2.17 and Preview news endpoints with what the API does (#757)

* Align the 2.17 News reference with what the API does

Switches the lists to page-number paging and documents page and per_page, gives update its own request schema, documents the 400s and 404s the API returns, and adds the accepted OAuth scopes to each operation.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Say that only optional fields keep their value on a news item update

sender_id is required on update, so the request description now limits "keeps its current value when left out" to the optional fields.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Port the News changes to Preview

Applies the same 2.17 changes to the Preview spec, merged operation by operation so Preview-only content stays, such as the `{id}` path parameter name. The next version is cut from Preview, and the Preview SDKs are generated from it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Document the reaction limits, label order and two more news item 400s

Reactions accept at most five entries, each a single Unicode code point. Labels sort with uppercase letters first, and newsfeed_assignment.published_at goes back to null when an item returns to draft. Create now shows the invalid-state 400, and create and update show the future published_at 400. Applied to 2.17 and Preview.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Say that a create with too many reactions still saves a draft

A create request with more than five reactions returns a 400 after the news item is already saved as a draft, with no reactions or cover image. The cover image check matches `.png` or `.jpg` anywhere in the URL and is case-sensitive, so the description now says "contain" and names the rejected `.jpeg` and `.PNG`. The response's reactions list also says that an update leaving out `reactions` disables them without clearing the list. Applied to 2.17 and Preview.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: markdennis22