ACP: Missing SessionUpdate notifications (user_message_chunk + tool_call_start/end)

[Leoyzen]

Missing Feature: ACP SessionUpdate notifications not emitted

Found by: test_acp_session_updates.py (2 xfail)

Missing notifications:

1. user_message_chunk — AgentPool ACP server does not emit user_message_chunk SessionUpdate notification when user sends a message
2. tool_call_start / tool_call_end — AgentPool does not emit tool_call_start/end SessionUpdate notifications when a tool is invoked

Location: src/agentpool_server/acp_server/event_converter.py — SessionUpdate conversion logic

Impact: ACP clients (e.g., Zed) cannot see user message echo or tool call progress via SessionUpdate notifications.

Tests:

• tests/e2e/test_acp_session_updates.py (user_message_chunk)
• tests/e2e/test_acp_session_updates.py::test_tool_call_start_end_notifications

Severity: Low-Medium — these are optional ACP v1 notifications; 9 other SessionUpdate variants ARE emitted correctly

[Leoyzen]

Update #196: user_message_chunk notified via _emit_user_message_chunks in ACP handler, emitting UserMessageChunk before prompt processing.tool_call_start / tool_call_end still not implemented — ACP subprocess tool call hangs. Keeping issue open for that portion. Refer to PR #200 for the partial fix (merge commit bdedd4517).

[Million-mo]

Fixed by #200 (merged directly to main).

What was fixed: ACP server now emits user_message_chunk SessionUpdate notifications. The fix adds a UserMessageChunk type and _emit_user_message_chunks in the ACP handler, so clients receive real-time notifications when user messages are processed.

Note: The issue title also mentions tool_call_start/end notifications. The user_message_chunk part is confirmed fixed in #200. If tool_call_start/end notifications are still missing, please open a new issue to track that separately.