Prefer RuntimeEvent model history when complete

[likun666661]

Summary

• thread optional runtimeContext through BackendSendInput, InvocationRequest, FlowInput, RuntimeRunner, AiSdkFlow, and SessionManager
• have AgentRun load prior per-run RuntimeEvents and use them only when their text projection covers the legacy prior user/assistant context
• make AiSdkBackend prefer RuntimeEvent-derived text history with StoredMessage fallback, keeping the current user message appended once
• add coverage for projection policy, backend preference/fallback, flow/runner propagation, and production SessionManager loading

Verification

• npm run build --workspace packages/core
• npm run typecheck --workspace packages/runtime
• npm run test --workspace packages/runtime -- --runInBand (405 tests)
• npm run test --workspace packages/storage -- --runInBand (79 tests)
• git diff --check

Rive/Codex DAG

• workflow: wfrun_52dd2a5bcb344693986cda23a564efb8
• scheduler: sched_d636efa23fb1428da7578f0fc11aa27d
• nodes: baseline contract, implementation, verify/fix, final review all accepted