session.resume fails with 'Session file is corrupted' when attachments field is null

[bsauvage1]

Bug Report

Summary

Resuming a session via session.resume fails with a JSON-RPC error when the saved session file has attachments: null. The SDK now expects attachments to be an array, but previously saved sessions store null.

Error

JSON-RPC Error -32603: Request session.resume failed with message:
Session file is corrupted (line 24: data.attachments: Expected array, received null)

Steps to Reproduce

1. Create a session (with SDK version that stores attachments: null in the session file)
2. Save the session ID
3. On a new run, call session.resume with the saved session ID
4. Observe the error above

Expected Behavior

Either:

• The SDK accepts null as a valid (empty) value for attachments during resume (backwards-compatible), or
• A more graceful error that allows the caller to detect the corruption and fall back to starting a new session

Actual Behavior

Resume throws a hard JSON-RPC -32603 error that cannot be easily distinguished from other fatal errors, forcing the caller to start a fresh session without a clean signal.

Environment

• github-copilot-sdk version: 0.1.32
• Python: 3.12.3
• OS: Linux (Ubuntu 24.04)

Suggested Fix

Treat null as equivalent to [] for the attachments field during session deserialization, to maintain backwards compatibility with older session files.