Add per-session mutex locking to prevent race conditions

Commands executing concurrently within the same session could cause
race conditions and interleaved output. This adds per-session mutexes
using async-mutex to serialize execution within each session while
preserving parallelism across different sessions.

The withSession API enables atomic multi-command operations where the
lock is held for the entire callback. Background streaming mode allows
long-running processes like servers to release the lock after startup
so other commands can proceed.

Author: ghostwriternr

.changeset/session-locking.md

@@ -0,0 +1,5 @@
+---
+'@cloudflare/sandbox': patch
+---
+
+Add per-session mutex locking to prevent concurrent command execution race conditions

packages/sandbox-container/src/core/types.ts

@@ -56,6 +56,18 @@ export interface ServiceError {
   details?: Record<string, unknown>;
 }
 
+/**
+ * Helper functions to construct ServiceResult with proper typing.
+ * Use these instead of manual object construction to avoid type casts.
+ */
+export function serviceSuccess<T>(data: T): ServiceResult<T> {
+  return { success: true, data } as ServiceResult<T>;
+}
+
+export function serviceError<T>(error: ServiceError): ServiceResult<T> {
+  return { success: false, error } as ServiceResult<T>;
+}
+
 // Handler error response structure - matches BaseHandler.createErrorResponse()
 export interface HandlerErrorResponse {
   success: false;