fix: prevent concurrent subagent file write conflicts in sandbox tools (#1714)

* fix: prevent concurrent subagent file write conflicts

Serialize same-path str_replace operations in sandbox tools

Guard AioSandbox write_file/update_file with the existing sandbox lock

Add regression tests for concurrent str_replace and append races

Verify with backend full tests and ruff lint checks

* fix(sandbox): Fix the concurrency issue of file operations on the same path in isolated sandboxes.

Ensure that different sandbox instances use independent locks for file operations on the same virtual path to avoid concurrency conflicts. Change the lock key from a single path to a composite key of (sandbox.id, path), and add tests to verify the concurrent safety of isolated sandboxes.

* feat(sandbox): Extract file operation lock logic to standalone module and fix concurrency issues

Extract file operation lock related logic from tools.py into a separate file_operation_lock.py module.
Fix data race issues during concurrent str_replace and write_file operations.

backend/CLAUDE.md

@@ -232,7 +232,7 @@ Proxied through nginx: `/api/langgraph/*` → LangGraph, all other `/api/*` →
 - `ls` - Directory listing (tree format, max 2 levels)
 - `read_file` - Read file contents with optional line range
 - `write_file` - Write/append to files, creates directories
-- `str_replace` - Substring replacement (single or all occurrences)
+- `str_replace` - Substring replacement (single or all occurrences); same-path serialization is scoped to `(sandbox.id, path)` so isolated sandboxes do not contend on identical virtual paths inside one process
 
 ### Subagent System (`packages/harness/deerflow/subagents/`)