chore(docker): Refactor sandbox state management and improve Docker integration (#1068)

* Refactor sandbox state management and improve Docker integration

- Removed FileSandboxStateStore and SandboxStateStore classes for a cleaner architecture.
- Enhanced LocalContainerBackend to handle port allocation retries and introduced environment variable support for sandbox host configuration.
- Updated Paths class to include host_base_dir for Docker volume mounts and ensured proper permissions for sandbox directories.
- Modified ExtensionsConfig to improve error handling when loading configuration files and adjusted environment variable resolution.
- Updated sandbox configuration to include a replicas option for managing concurrent sandbox containers.
- Improved logging and context management in SandboxMiddleware for better sandbox lifecycle handling.
- Enhanced network port allocation logic to bind to 0.0.0.0 for compatibility with Docker.
- Updated Docker Compose files to ensure proper volume management and environment variable configuration.
- Created scripts to ensure necessary configuration files are present before starting services.
- Cleaned up unused MCP server configurations in extensions_config.example.json.

* Address Copilot review suggestions from PR #1068 (#9)

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

config.example.yaml

@@ -201,9 +201,6 @@ sandbox:
 # sandbox:
 #   use: src.community.aio_sandbox:AioSandboxProvider
 #
-#   # Optional: Use existing sandbox at this URL (no container will be started)
-#   # base_url: http://localhost:8080
-#
 #   # Optional: Container image to use (works with both Docker and Apple Container)
 #   # Default: enterprise-public-cn-beijing.cr.volces.com/vefaas-public/all-in-one-sandbox:latest
 #   # Recommended: enterprise-public-cn-beijing.cr.volces.com/vefaas-public/all-in-one-sandbox:latest (works on both x86_64 and arm64)
@@ -211,9 +208,11 @@ sandbox:
 #
 #   # Optional: Base port for sandbox containers (default: 8080)
 #   # port: 8080
-#
-#   # Optional: Whether to automatically start Docker container (default: true)
-#   # auto_start: true
+
+#   # Optional: Maximum number of concurrent sandbox containers (default: 3)
+#   # When the limit is reached the least-recently-used sandbox is evicted to
+#   # make room for new ones. Use a positive integer here; omit this field to use the default.
+#   # replicas: 3
 #
 #   # Optional: Prefix for container names (default: deer-flow-sandbox)
 #   # container_prefix: deer-flow-sandbox
@@ -348,7 +347,6 @@ memory:
   injection_enabled: true # Whether to inject memory into system prompt
   max_injection_tokens: 2000 # Maximum tokens for memory injection
 
-
 # ============================================================================
 # Checkpointer Configuration
 # ============================================================================
@@ -373,9 +371,9 @@ memory:
 #   type: memory
 #
 # SQLite (file-based, single-process):
-# checkpointer:
-#   type: sqlite
-#   connection_string: checkpoints.db
+checkpointer:
+  type: sqlite
+  connection_string: checkpoints.db
 #
 # PostgreSQL (multi-process, production):
 # checkpointer: