fix: narrow pinterest bot-detection entry to avoid blocking Pinterest's in-app browser (#4636)

* fix: narrow pinterest bot-detection entry to avoid blocking Pinterest's in-app browser

DEFAULT_BLOCKED_UA_STRS matched the bare substring 'pinterest', which
also matches the Pinterest app's own in-app browser UA (e.g. "...
Mobile/15D100 [Pinterest/iOS]"). isBlockedUA does a case-insensitive
substring match against the whole UA string, so real Pinterest-app
users were silently classified as bots and excluded from analytics.

Narrowed the entry to 'pinterestbot', which still matches the crawler's
primary UA ("...Pinterestbot/1.0; +http://www.pinterest.com/bot.html)")
without matching the in-app browser. The crawler's other UA variant
("Pinterest/0.2 (+https://www.pinterest.com/bot.html)") remains covered
by the existing generic 'bot.htm' entry, so no bot-detection coverage
is lost.

Addresses one specific sub-bug reported in #83413 (the pinterest
false-positive) -- not the broader per-platform UA-detection gap in
that issue (Instagram/LinkedIn/TikTok/etc.), which spans multiple
repos and implementations and needs separate, larger work.

* chore: add changeset for pinterest bot-detection fix

Requested by @dustinbyrne in PR review -- RELEASING.md requires a
changeset for any change that will be published.