fix: ACP improvements - single-line retry, variable name, and cancel optimization

[dascard]

Summary

This PR includes several ACP-related fixes and improvements:

1. Fix single-line retry/edit returning empty content
2. Fix ACP client variable name inconsistency
3. Use cancel_session instead of stopping ACP client
4. Reset sidebar state after cancellation

Problems Fixed

1. Single-line retry/edit broken

When a user message is a single line, get_current_user_request_block() returns empty content because line 755 unconditionally removes the last element:

content_lines = vim.list_slice(content_lines, 1, #content_lines - 1)

For single-line messages, this results in an empty table.

2. ACP client stop not working
   cancel_inflight_request() calls get_registered_acp_clients() which returns M._acp_clients, but register_acp_client() stores clients in M.acp_clients. This mismatch causes the stop function to never find any clients.

3. Full client stop causes reconnection delay
   Previously, cancellation would stop the entire ACP client process, requiring a new connection for subsequent requests.

4. UI stuck in "generating" state after cancel
   After cancelling, the sidebar state was not properly reset.

Solution
Only slice content_lines when there are multiple lines
Use acp_clients consistently (not _acp_clients)
Use cancel_session() to cancel the current session while keeping the connection alive
Reset current_state, scroll, and clear state extmarks after cancellation
Testing
Send a single-line message → retry/edit now works
Cancel during generation → UI properly resets
Send new message after cancel → responds without reconnection delay

[PeterCardenas]

part of this was superseded by #2847.