fix(sandbox): resolve paths in read_file/write_file content for LocalSandbox (#1935)

* fix(sandbox): resolve paths in read_file/write_file content for LocalSandbox

In LocalSandbox mode, read_file and write_file now transform
container paths in file content, matching the path handling
behavior of bash tool.

- write_file: resolves virtual paths in content to system paths
  before writing, so scripts with /mnt/user-data paths work
  when executed
- read_file: reverse-resolves system paths back to virtual
  paths in returned content for consistency

This fixes scenarios where agents write Python scripts with
virtual paths, then execute them via bash tool expecting the
paths to work.

Fixes #1778

* fix(sandbox): address Copilot review — dedicated content resolver + forward-slash safety + tests

- Extract _resolve_paths_in_content() separate from _resolve_paths_in_command()
  to decouple file-content path resolution from shell-command parsing
- Normalize resolved paths to forward slashes to avoid Windows backslash
  escape issues in source files (e.g. \U in Python string literals)
- Add 4 focused tests: write resolves content, forward-slash guarantee,
  read reverse-resolves content, and write→read roundtrip

* style: fix ruff lint — remove extraneous f-string prefix

* fix(sandbox): only reverse-resolve paths in agent-written files

read_file previously applied _reverse_resolve_paths_in_output to ALL
file content, which could silently rewrite paths in user uploads and
external tool output (Willem Jiang review on #1935).

Now tracks files written through write_file in _agent_written_paths.
Only those files get reverse-resolved on read. Non-agent files are
returned as-is.

---------

Co-authored-by: JasonOA888 <JasonOA888@users.noreply.github.com>

backend/tests/test_local_sandbox_provider_mounts.py

@@ -363,6 +363,98 @@ class TestLocalSandboxProviderMounts:
 
         assert [m.container_path for m in provider._path_mappings] == ["/mnt/skills"]
 
+    def test_write_file_resolves_container_paths_in_content(self, tmp_path):
+        """write_file should replace container paths in file content with local paths."""
+        data_dir = tmp_path / "data"
+        data_dir.mkdir()
+
+        sandbox = LocalSandbox(
+            "test",
+            [
+                PathMapping(container_path="/mnt/data", local_path=str(data_dir)),
+            ],
+        )
+        sandbox.write_file(
+            "/mnt/data/script.py",
+            'import pathlib\npath = "/mnt/data/output"\nprint(path)',
+        )
+        written = (data_dir / "script.py").read_text()
+        # Container path should be resolved to local path (forward slashes)
+        assert str(data_dir).replace("\\", "/") in written
+        assert "/mnt/data/output" not in written
+
+    def test_write_file_uses_forward_slashes_on_windows_paths(self, tmp_path):
+        """Resolved paths in content should always use forward slashes."""
+        data_dir = tmp_path / "data"
+        data_dir.mkdir()
+
+        sandbox = LocalSandbox(
+            "test",
+            [
+                PathMapping(container_path="/mnt/data", local_path=str(data_dir)),
+            ],
+        )
+        sandbox.write_file(
+            "/mnt/data/config.py",
+            'DATA_DIR = "/mnt/data/files"',
+        )
+        written = (data_dir / "config.py").read_text()
+        # Must not contain backslashes that could break escape sequences
+        assert "\\" not in written.split("DATA_DIR = ")[1].split("\n")[0]
+
+    def test_read_file_reverse_resolves_local_paths_in_agent_written_files(self, tmp_path):
+        """read_file should convert local paths back to container paths in agent-written files."""
+        data_dir = tmp_path / "data"
+        data_dir.mkdir()
+
+        sandbox = LocalSandbox(
+            "test",
+            [
+                PathMapping(container_path="/mnt/data", local_path=str(data_dir)),
+            ],
+        )
+        # Use write_file so the path is tracked as agent-written
+        sandbox.write_file("/mnt/data/info.txt", "File located at: /mnt/data/info.txt")
+
+        content = sandbox.read_file("/mnt/data/info.txt")
+        assert "/mnt/data/info.txt" in content
+
+    def test_read_file_does_not_reverse_resolve_non_agent_files(self, tmp_path):
+        """read_file should NOT rewrite paths in user-uploaded or external files."""
+        data_dir = tmp_path / "data"
+        data_dir.mkdir()
+
+        sandbox = LocalSandbox(
+            "test",
+            [
+                PathMapping(container_path="/mnt/data", local_path=str(data_dir)),
+            ],
+        )
+        # Write directly to filesystem (simulates user upload or external tool output)
+        local_path = str(data_dir).replace("\\", "/")
+        (data_dir / "config.yml").write_text(f"output_dir: {local_path}/outputs")
+
+        content = sandbox.read_file("/mnt/data/config.yml")
+        # Content should be returned as-is, NOT reverse-resolved
+        assert local_path in content
+
+    def test_write_then_read_roundtrip(self, tmp_path):
+        """Container paths survive a write → read roundtrip."""
+        data_dir = tmp_path / "data"
+        data_dir.mkdir()
+
+        sandbox = LocalSandbox(
+            "test",
+            [
+                PathMapping(container_path="/mnt/data", local_path=str(data_dir)),
+            ],
+        )
+        original = 'cfg = {"path": "/mnt/data/config.json", "flag": true}'
+        sandbox.write_file("/mnt/data/settings.py", original)
+        result = sandbox.read_file("/mnt/data/settings.py")
+        # The container path should be preserved through roundtrip
+        assert "/mnt/data/config.json" in result
+
     def test_setup_path_mappings_normalizes_container_path_trailing_slash(self, tmp_path):
         skills_dir = tmp_path / "skills"
         skills_dir.mkdir()