refactor(session): destroy SessionRunState facade

[kitlangton]

Summary

• Removed makeRuntime and assertNotBusy facade from SessionRunState
• Converted the delete-message handler in server/routes/session.ts to wrap the entire body in Effect.gen, yielding both SessionRunState.Service and Session.Service from context
• Updated session-actions.test.ts to make the session actually busy via ensureRunning instead of spying on the deleted facade

Verification

• bun run typecheck — clean
• bun run test test/server/session-actions.test.ts — 2/2