feat(helexa-acp): wire ACP agent loop for text-only conversations

Stage 2 lands the agent loop on top of the Stage 1 scaffold: session
state with per-session cancellation, a system-prompt builder honouring
HELEXA_ACP_SYSTEM_PROMPT_PATH / system_prompt_path TOML, and handlers
for initialize / session/new / session/prompt / session/cancel that
stream provider output back as session/update notifications. Verified
end-to-end against cortex from Zed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/helexa-acp/src/provider/openai_chat.rs

@@ -15,7 +15,7 @@ use tokio_util::sync::CancellationToken;
 
 use super::{
     CompletionEvent, CompletionRequest, Message, MessageContent, ModelInfo, Provider, Role,
-    ToolCall, ToolSpec, UsageStats,
+    ToolSpec, UsageStats,
 };
 use crate::config::EndpointConfig;
 
@@ -126,6 +126,7 @@ impl Provider for OpenAIChatProvider {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::provider::ToolCall;
     use futures::stream;
     use url::Url;