Fix sandbox file watcher hanging on macOS temp directories

[pascalwhoop]

Problem

When sandbox is enabled, the Claude CLI's file watcher attempts to monitor macOS temp directories (/var/folders/...) and fails on:

1. Socket files - EOPNOTSUPP errors on .sock files from VSCode, Docker, SSH-askpass
2. Temp directories - EINTR errors on directories like python-test-discovery-*

These unhandled promise rejections cause the SDK to hang indefinitely instead of failing fast with a clear error.

Solution

This PR adds detection and handling for sandbox file watcher errors:

1. New Error Class: SandboxFileWatcherError - Provides clear error messages with workaround suggestions
2. Stderr Monitoring: When sandbox is enabled, stderr is always piped to detect file watcher errors
3. Error Detection: Regex pattern matches EOPNOTSUPP and EINTR errors from CLI stderr
4. Graceful Failure: When detected, the SDK terminates the CLI process and raises SandboxFileWatcherError instead of hanging

⚠️ The real fix would be in the nodejs code but I don't think that is open source? I couldn't find that repository anyways :-/

Changes

• Added SandboxFileWatcherError exception class
• Modified SubprocessCLITransport to detect file watcher errors from stderr
• Always pipe stderr when sandbox is enabled (for error detection)
• Added tests for error class and regex pattern detection

Testing

• ✅ All existing SDK tests pass
• ✅ Integration tests with sandbox enabled pass (6/6 tests)
• ✅ Error detection pattern correctly matches error messages
• ✅ Process terminates immediately when error detected (no hanging)

Example Error Message

SandboxFileWatcherError: Sandbox file watcher failed on '/var/folders/.../vscode-git-*.sock' (EOPNOTSUPP).
This is a known issue with the Claude CLI's sandbox on macOS.
Workarounds: 1) Disable sandbox (sandbox=None), 2) Run in a container with a clean /tmp directory.

Fixes the issue where sandbox-enabled SDK calls would hang forever when VSCode, Docker, or other apps have socket files in system temp directories.