Streamable Transport Session Lifecycle Owns Transport Continuity

We decided to introduce Streamable Transport Session Lifecycle in src/transport/http/streamableSessionLifecycle.ts as the transport-level owner for Streamable HTTP session creation, active lookup, restoration, initialize recovery, post-initialize bookkeeping, and cleanup outcomes. Routes remain HTTP adapters around SDK request handling, while active-transport cleanup stays behind ServerManager.disconnectTransport() so disconnect/delete also releases related Template Server Instance memberships into the existing idle-cleanup path. The first slice preserves current HTTP error mapping and SDK-internal restoration behavior while exposing richer structured lifecycle results for tests and diagnostics.