Shape the result into a Data-Snapshot-compatible payload so ProcessAgentArtifacts
can persist it as a Search Result Set artifact. See Section 6.3 of the plan.
Returns undefined when there is nothing to persist.
Main entry point. Returns a filled-in AgentPreExecutionRAGResult when the agent
has any active PreExecution/Both scopes; otherwise returns null so callers can
skip injection entirely.
Orchestrator for agent pre-execution RAG. Stateless — create a fresh instance per call.