feat: truncate oversized event payloads in session response (#524)

RAG tool results can return KB document content exceeding 1MB per event,
causing the frontend TraceSurface to run out of memory when loading large
sessions. Truncate tool_result and observation payloads at 1MB in the
get_session endpoint, marking truncated events with _truncated: true.

The LLM context builder reads from a separate content-only store and
is unaffected by this truncation.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>