feat: support custom session_id via ACP _meta field in new_session()

[michaelsinghk]

Summary

Allows ACP clients to pass a deterministic session_id through the standard ACP _meta extensibility field in session/new requests. When _meta contains a session_id key, the agent uses it instead of generating a random UUID.

Motivation

ACP clients (like Virtual Team orchestrators or CI pipelines) need deterministic session IDs to:

• Persist and resume conversations across process restarts
• Map agent conversations to external task IDs
• Keep the ACP process alive while switching between multiple role-specific conversations

Currently, session/new always generates a random UUID, giving the client no control over the session identity.

Change

File: openhands_cli/acp_impl/agent/base_agent.py — new_session() method

# Before:
else:
    session_id = str(uuid.uuid4())

# After:
else:
    _meta = _kwargs.get("_meta") or {}
    custom_session_id = _meta.get("session_id")
    session_id = str(custom_session_id) if custom_session_id else str(uuid.uuid4())

7 lines inserted, 1 deleted. Purely additive — all existing behavior is preserved when _meta is absent. The _meta field is already part of the ACP protocol spec and collected by the NewSessionRequest schema.

Usage

ACP clients can now send:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "session/new",
  "params": {
    "cwd": "/workspace",
    "_meta": {
      "session_id": "my-task-id-001"
    }
  }
}

Testing

• Existing behavior unchanged when _meta is absent (random UUID continues)
• Manual test confirmed: custom session_id passed via _meta is used as the conversation ID
• Conversation persistence dir created at ~/.openhands/conversations/{custom_id}.hex/ as expected