Adjust AI messages trimming logic to only keep the latest message

[sentrivana]

Adjust the logic in

sentry-python/sentry_sdk/ai/utils.py

Line 648 in 1970486

def truncate_messages_by_size(

to always only include the last (newest) message rather than all that fit into the 20kB limit.

All our integrations should use the function linked above already, so changing the logic in it should be all that's necessary (and adjusting the tests).

Context (sorry, Sentry employees only): https://sentry.slack.com/archives/C06KVE7V0DA/p1768578892574999?thread_ts=1768311204.311329&cid=C06KVE7V0DA

[linear]

PY-2037