fix(app): keep terminal mounted when switching session tabs in a workspace (#34852)

Co-authored-by: Brendan Allan <git@brendonovich.dev>

packages/app/src/utils/server-errors.ts

@@ -42,6 +42,20 @@ function unwrapNamedError(error: unknown): unknown {
   return error
 }
 
+// Client-synthesized session not-found errors share one constructor and
+// predicate so the message contract cannot drift between the sync store
+// (server-session.ts), the route lineage (session-lineage.ts), and the
+// not-found fallback matching (session.tsx).
+const sessionNotFoundMessage = (sessionID: string) => `Session not found: ${sessionID}`
+
+export function sessionNotFoundError(sessionID: string) {
+  return new Error(sessionNotFoundMessage(sessionID))
+}
+
+export function isLocalSessionNotFoundError(error: unknown, sessionID: string) {
+  return error instanceof Error && error.message === sessionNotFoundMessage(sessionID)
+}
+
 export function isSessionNotFoundError(error: unknown, sessionID: string) {
   const unwrapped = unwrapNamedError(error)
   if (typeof unwrapped !== "object" || unwrapped === null) return false