docs(sdk): document ACP remote conversation routing

[simonrosenberg]

Summary

• document how ACPAgent works with RemoteConversation and remote agent-server deployments
• note that ACP-backed remote conversations use the ACP-capable /api/acp/conversations surface for create/read/count operations
• document the explicit attach-by-conversation_id mismatch behavior for ACP conversations
• add a pointer from the API sandbox guide to the ACP remote-runtime example

Context

This docs PR accompanies OpenHands/software-agent-sdk PR #2465.

Related SDK follow-up issue: OpenHands/software-agent-sdk#2469

Testing

• git diff --check

[all-hands-bot]

🟢 Good taste - Clean separation of concerns, accurate documentation.

Verified all claims against the source code in feat/acp-remote-runtime-acp-endpoints:

✅ ACPAgent + RemoteConversation routing logic exists and works as documented
✅ /api/acp/conversations endpoints exist (POST, GET, search, batch get, count)
✅ Conversation creation/info reads use ACP paths, actions use legacy paths
✅ Mismatch detection prevents attaching with wrong agent type
✅ Example file 09_acp_agent_with_remote_runtime.py exists and demonstrates the feature

KEY INSIGHT: The dual-path architecture (info via ACP, actions via legacy) is elegant - it isolates schema-sensitive contract changes while keeping execution flow untouched. No special cases, no complexity creep.