feat(acp-nats): add integration test suite for Bridge, handlers, and workflows

- Add tests/ module with agent_handlers, builders, prefix, and workflows
- Cover all 9 agent handlers with success and error paths
- Test CancelledSessions, prompt waiter resolution, session.ready tasks

Signed-off-by: Yordis Prieto <yordis.prieto@gmail.com>

Author: yordis

rsworkspace/crates/acp-nats/src/agent/cancel.rs

@@ -1,7 +1,9 @@
 use super::Bridge;
 use crate::nats::{self, FlushClient, PublishClient, RequestClient, agent};
 use crate::session_id::AcpSessionId;
-use agent_client_protocol::{CancelNotification, Error, ErrorCode, Result};
+use agent_client_protocol::{
+    CancelNotification, Error, ErrorCode, PromptResponse, Result, StopReason,
+};
 use tracing::{info, instrument, warn};
 use trogon_std::time::GetElapsed;
 
@@ -62,7 +64,10 @@ pub async fn handle<N: RequestClient + PublishClient + FlushClient, C: GetElapse
         .mark_cancelled(args.session_id.clone(), &bridge.clock);
     bridge
         .pending_session_prompt_responses
-        .cancel_waiter_for_session(&args.session_id, Err("Session was cancelled".to_string()));
+        .cancel_waiter_for_session(
+            &args.session_id,
+            Ok(PromptResponse::new(StopReason::Cancelled)),
+        );
 
     bridge.metrics.record_request(
         "cancel",

rsworkspace/crates/acp-nats/src/agent/prompt.rs

@@ -89,12 +89,9 @@ pub async fn handle<N: RequestClient + PublishClient + FlushClient, C: GetElapse
             .take_if_cancelled(&args.session_id, &bridge.clock)
             .is_some()
         {
-            bridge
-                .metrics
-                .record_error("prompt", "session_cancelled_preflight");
-            return Err(Error::new(
-                ErrorCode::InvalidParams.into(),
-                format!("Session {} was recently cancelled", args.session_id),
+            info!(session_id = %args.session_id, "Prompt pre-flight: session already cancelled");
+            return Ok(PromptResponse::new(
+                agent_client_protocol::StopReason::Cancelled,
             ));
         }
 

rsworkspace/crates/acp-nats/src/lib.rs

@@ -12,6 +12,8 @@ pub mod prompt_slot_counter;
 pub mod session_id;
 pub mod subject_token_violation;
 pub(crate) mod telemetry;
+#[cfg(test)]
+mod tests;
 
 pub use acp_prefix::{AcpPrefix, AcpPrefixError};
 pub use agent::Bridge;