vMCP: bind sessions to the client, acting agent, and delegated-access marker, not just the user #6888

Problem: a vMCP session is bound to the user only. Once delegated tokens reach the gateway, tokens from different clients, or from different agents acting for the same user, could share a session and its cached state.

Change: bind each session to the user, the client_id, the actor's issuer and subject (act.iss, act.sub), and the presence of the delegated-access marker (#6404). A request whose token differs on any of these is rejected for that session, on every method including list methods.

Blocked by: #6779 (enforce the existing binding on list methods first). Client and marker binding are needed once delegated clients other than one configured broker reach the gateway; actor binding once agents carry their own identity (act).