save assistant messages as EventNewAssistantMessage instead of EventNewToken

[x2d7]

"Minor version numbers are for when you're ashamed of the changes."

Summary

Session was appending the collected assistant response to chat history as NewEventNewToken instead of NewEventNewAssistantMessage. Since the SyncInput contract explicitly states that streaming-only events (including EventNewToken) should not be synced to client params, providers were silently dropping the assistant's turns — making the model appear to have no memory of its own responses.

Changes

• handleCompletionEnd now appends NewEventNewAssistantMessage instead of NewEventNewToken
• SyncInput and AppendEvent contracts documented explicitly in types.go and helpers.go
• Test TestSession_CollectsTokens updated to assert on EventNewAssistantMessage

Closes #26