fix: namespace sandbox temp paths by sandbox ID to prevent cross-session collisions

DifyCli and AppAssets used hardcoded absolute paths under /tmp/.dify/,
causing concurrent sandbox sessions on the same host (e.g. SSH provider)
to overwrite each other's config files and CLI binaries.

- Add Sandbox.id property (uuid4 hex) as a stable, path-safe identifier
  independent of provider-specific environment IDs
- Convert DifyCli/AppAssets from static constants to per-sandbox instances
  namespaced under /tmp/.dify/{sandbox.id}/
- Replace all vm.metadata.id references with sandbox.id
- Replace upload_file with heredoc-based pipeline step in session.py to
  reduce round-trips

api/core/sandbox/entities/config.py

@@ -2,18 +2,45 @@ from typing import Final
 
 
 class DifyCli:
-    """Dify CLI constants (absolute path - hidden in /tmp, not in sandbox workdir)"""
+    """Per-sandbox Dify CLI paths, namespaced under ``/tmp/.dify/{env_id}``.
 
-    ROOT: Final[str] = "/tmp/.dify"
-    PATH: Final[str] = "/tmp/.dify/bin/dify"
-    PATH_PATTERN: Final[str] = "dify-cli-{os}-{arch}"
+    Every sandbox environment gets its own directory tree so that
+    concurrent sessions on the same host (e.g. SSH provider) never
+    collide on config files or CLI binaries.
+
+    Class-level constants (``CONFIG_FILENAME``, ``PATH_PATTERN``) are
+    safe to share; all path attributes are instance-level and derived
+    from the ``env_id`` passed at construction time.
+    """
+
+    # --- class-level constants (no path component) ---
     CONFIG_FILENAME: Final[str] = ".dify_cli.json"
-    TOOLS_ROOT: Final[str] = "/tmp/.dify/tools"
-    GLOBAL_TOOLS_PATH: Final[str] = "/tmp/.dify/tools/global"
+    PATH_PATTERN: Final[str] = "dify-cli-{os}-{arch}"
+
+    # --- instance attributes ---
+    root: str
+    bin_path: str
+    tools_root: str
+    global_tools_path: str
+
+    def __init__(self, env_id: str) -> None:
+        self.root = f"/tmp/.dify/{env_id}"
+        self.bin_path = f"{self.root}/bin/dify"
+        self.tools_root = f"{self.root}/tools"
+        self.global_tools_path = f"{self.root}/tools/global"
 
 
 class AppAssets:
-    """App Assets constants (relative path - stays in sandbox workdir)"""
+    """App Assets constants.
+
+    ``PATH`` is a relative path resolved by each provider against its
+    own workspace root — already isolated.  ``zip_path`` is an absolute
+    temp path and must be namespaced per environment to avoid collisions.
+    """
 
     PATH: Final[str] = "skills"
-    ZIP_PATH: Final[str] = "/tmp/assets.zip"
+
+    zip_path: str
+
+    def __init__(self, env_id: str) -> None:
+        self.zip_path = f"/tmp/.dify/{env_id}/assets.zip"