Show where an AI agent conversation's model call history changed

[wu-sheng]

Show where an AI agent conversation's model call history changed

• If this is non-trivial feature, paste the links/URLs to the design doc.
• Update the documentation to include this new feature.
• Tests(including UT, IT, E2E) are added to verify the new feature.
• If it's UI related, attach the screenshots below.

This follows apache/skywalking-ai-sessionizer#77, which the asz.view format defines. The Sessionizer now compares
each model call's history with the call before it in its stream. Where the history changed, it adds a
history.change node just before the call, under the same parent, at the call's first record. The rules are on the
Sessionizer's Session Flow page, "Where a call's history changed".

The OAP mirrors two rules of the document:

• A card carries no content. It stands on its call's first record, but what it says is in its attrs, so
  ConversationViewBuilder.carriesContent lists history.change beside llm.call. Without it, the card took the
  call's text.
• A card comes just before its call. At one record, ConversationFold.compare orders a card by the id of the
  call it names, call/ and the card's key, and before that call, as the Sessionizer's Before does. By its own id,
  history/ sorts after call/.

Tests.

• history-forget is the Sessionizer's scenario of that name, built and parsed by the Sessionizer. The document the
  OAP builds from its files and round equals asz conversation -json, key for key and in order. The test also
  checks the card's place among its siblings and that it carries no text.

• CardOrderTest mirrors the Sessionizer's own order test: four nodes at one record, given in every rotation.

• Each rule was proven by reverting it and seeing its test fail.

• Update the CHANGES log.