Expose used token information after compaction

[DonJayamanne]

Today we get the used token information after messages,
& the SDK also exposes an API to compact conversations.
However once conversation is compacted, we do not get any details of the current (post compaction) token usage.

Suggestion is to expose this information so we know how much context window is available after compaction

[stephentoub]

This has been addressed. A change has been merged into the CLI that exposes the post-compaction context window details as part of the results and also ensure a session.usage_info is emitted after background compaction.