fix: tolerate context-likes without user_content or session in record_agent_invocation

Use getattr for ctx.user_content and ctx.session.events when calling
_record_agent_metrics so that test doubles, partial migrations, and
external embedders that pass minimal InvocationContext-like objects
don't crash with AttributeError on the metrics call.

Change-Id: I7d54252dd222f3796e6ccdc13d4844a177b65e7d