fix(openclaw-plugin): tolerate missing OV session during assemble

[marchpure]

Summary

• Treat OpenViking Session not found during OpenClaw context assemble as an empty/new session passthrough instead of a warning-level assemble failure.
• Reuse the same NOT_FOUND predicate for compact's existing missing-session skip path.

Why

For a brand-new OpenClaw session, assemble can run before afterTurn has captured any messages into OpenViking. The /context endpoint returns [NOT_FOUND]: Session not found in that lifecycle window. This is expected for a first turn and should not be logged as an assemble failure.

Verification

• node --check /root/work-openviking-api-fix/files/context-lifecycle-service.ts
• Targeted local runtime check against installed OpenViking plugin: fake getSessionContext() throws [NOT_FOUND]: Session not found; verified passthrough result, reason=session_not_found, and no warn log.