vmcp/session: expose a pluggable backendConnector on NewSessionFactory

[JAORMX]

Problem

session.NewSessionFactory(registry, opts...) at pkg/vmcp/session/factory.go:187 is the only public constructor for MultiSessionFactory. It hardcodes backend.NewHTTPConnector(registry) as the connector.

The internal plumbing for a custom connector exists:

• backendConnector type at factory.go:117 (unexported)
• newSessionFactoryWithConnector at factory.go:194 (unexported, appears test-only)
• backend.Session + backend.NewHTTPConnector live under pkg/vmcp/session/internal/backend/ — Go's internal path rule prevents external use.

Use case

Embedders whose gateway already resolves per-backend capability lists (for conflict resolution, ACL filtering, static catalog composition) need the session factory to present those pre-resolved capabilities instead of re-fetching them over HTTP at every session initialize. Similarly, embedders with stateless dispatch models do not want the factory to open N live MCP client connections per session.

Proposed API

Export the backendConnector interface (or lift it to pkg/vmcp/session/types next to MultiSession), and add a functional option:

func WithBackendConnector(conn BackendConnector) MultiSessionFactoryOption

When supplied, NewSessionFactory uses it instead of backend.NewHTTPConnector. The default stays unchanged.

Separately, expose backend.Session (or a narrower interface specifically for connector implementations) via pkg/vmcp/session/types so external implementations have a target type to return.

Scope

• Additive. No change to the existing HTTP-connector default.
• May require widening backend.Session to live outside internal/ — suggest moving just the interface, not the HTTP implementation.