[vMCP] Expose active backend sessions on health endpoint

Depends on #3866

Session lifecycle events and periodic session counts should be logged for operational debugging and compliance auditing when sessionManagementV2 is enabled.

Required logging

On session creation (CreateSession):

• Log at Info level with: session ID, backend count, backend names

On session termination/expiry (Terminate):

• Log at Info level with: session ID, backend count

Periodic active session summary (every 1 minute):

• Log at Info level with: total active session count, per-backend session counts

Implementation

• Manager.CreateSession(): log at Info when a session is fully created
• Manager.Terminate(): log at Info with backend count for MultiSessions; log placeholder termination separately
• Manager.StartPeriodicLogging(ctx, interval): background goroutine that calls internal logActiveSessions() on a ticker
• logActiveSessions(): iterates sessions via Range(), aggregates per-backend counts, logs summary
• Called from Server.Start() when SessionManagementV2 is enabled, with a 1-minute default interval

Acceptance Criteria

• Session creation is logged at Info with session ID, backend count, and backend names
• Session termination is logged at Info with session ID and backend count
• Active sessions are periodically logged at Info with total count and per-backend counts
• No session IDs or backend session IDs are exposed over the network (logging only, to operators)
• The periodic logger stops when the server context is cancelled
• The implementation degrades gracefully with distributed storage backends (logs a one-time warning)

RFC: THV-0038 — Session-scoped client lifecycle

Start with the session manager methods Manager.CreateSession(), Manager.Terminate(), Range(), and Server.Start(), then inspect the RFC and dependency issue #3866 for lifecycle context. Implement and verify creation, termination, periodic aggregation, cancellation, and distributed-storage warning behavior against the listed acceptance criteria.