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>

backend/src/utils/network.py

@@ -44,9 +44,13 @@ class PortAllocator:
         if port in self._reserved_ports:
             return False
 
+        # Bind to 0.0.0.0 (wildcard) rather than localhost so that the check
+        # mirrors exactly what Docker does.  Docker binds to 0.0.0.0:PORT;
+        # checking only 127.0.0.1 can falsely report a port as available even
+        # when Docker already occupies it on the wildcard address.
         with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
             try:
-                s.bind(("localhost", port))
+                s.bind(("0.0.0.0", port))
                 return True
             except OSError:
                 return False