fix(session): persist Event.NodeInfo through Vertex AI backend

[wolo-lab]

Problem

The Vertex AI session backend only round-tripped event fields that have a dedicated SessionEvent column (Content, Actions, EventMetadata). On write everything else was silently dropped, and on read it was never reconstructed: Output, NodeInfo, IsolationScope, RequestedInput and Routes.
This became correctness-critical once the workflow engine moved to event-scan resume reconstruction (ReconstructRunState / rehydrateCache), where NodeInfo.Path identifies the emitting node. A workflow paused and resumed against a Vertex-persisted session resumed incorrectly because NodeInfo was gone. The in-memory and SQL backends already preserved these fields; Vertex was the last gap.

Solution

Persist the full event in SessionEvent.raw_event (proto field 12, the same field adk-python uses) and prefer it on read, falling back to the legacy field-based reconstruction for events written before raw_event support.

• eventNeedsRawEvent — raw_event is attached only when the event carries one of the column-less fields (Output, NodeInfo, IsolationScope, RequestedInput, Routes). Plain events keep their existing wire format, so the recorded rpcreplay fixtures stay valid (none of them sets those fields).
• eventToRawEvent / eventFromRawEvent — JSON round-trip into structpb.Struct and back. On read, identity fields (ID, Timestamp, InvocationID, Author) are taken from the envelope.