AgentLoop is not stop-aware mid-iteration | Stop during a tool turn doesn't halt the loop

[ASahu16]

Goal

Make the agent loop honor a user Stop the same way plain chat does: when the user taps Stop during a tool-enabled turn, generation halts promptly, no further tools or model calls run, and the partial answer already streamed is kept as the assistant message.

Background

#2 added a working Stop for plain streaming chat (ChatNotifier.stop() → backend.stop() → native g_should_stop), and the partial reply is saved via onDone and tagged TraceOutcome.stopped.

#3 added the tool-calling agent loop (AgentLoop in lib/features/agent/application/agent_loop.dart), driven by ChatNotifier._runAgentTurn. Stop was intentionally left out of scope for the loop.

Current behavior (the bug)

ChatNotifier.stop() sets _stopRequested = true and calls backend.stop(). During an agent turn this only ends the current generation early — the await for in generateStream completes with partial output.
The loop then:

• proceeds to parseToolCall(partial) on the truncated output, and
• may execute a tool and/or run another iteration,

because AgentLoop never checks _stopRequested and _runAgentTurn passes no cancellation signal into it. So Stop doesn't actually stop the agent — it just truncates one step. The trace is also tagged done,
not stopped.

Expected behavior

• Stop during an agent turn ends the loop after the in-flight generation returns.
• No further tool executions or generations occur.
• If the loop was in answer mode (streaming the final answer), the partial text streamed so far is saved as the assistant message.
• If in tool mode (no answer streamed yet), end with whatever answer text exists (likely empty → no message, or a short "stopped" note).
• The I/O trace for the turn is tagged TraceOutcome.stopped.

Proposed implementation

1. AgentLoop — add an injected bool Function()? isCancelled (or a CancellationToken). Check it:
   - at the top of each iteration (before calling generateStream), and
   - immediately after each generation completes (before parsing/executing a tool).
   On cancel, return an AgentResult with answer = the accumulated answer-mode text (or empty), steps collected so far, and a new flag e.g. stopped: true.
2. ChatNotifier._runAgentTurn — pass isCancelled: () => _stopRequested into AgentLoop. On a stopped result, save the partial answer (if non-empty) as the assistant message and record the trace as
   TraceOutcome.stopped.
3. Optional: thread the cancellation into the generateStream await for so an in-flight generation is abandoned immediately rather than drained.

Affected files

• lib/features/agent/application/agent_loop.dart
• lib/features/chat/presentation/chat_notifier.dart (_runAgentTurn, stop())
• test/agent_loop_test.dart

Acceptance criteria

• Tapping Stop during a multi-tool agent turn halts after the current generation; no further generateStream/tool calls fire.
• A partial final answer is preserved as the assistant message.
• The turn's ModelIoTrace.outcome is stopped.
• Plain (tools-off) chat Stop behavior is unchanged.

Testing

• Unit test in test/agent_loop_test.dart: drive AgentLoop with isCancelled returning true after the first generation; assert it makes no further generateStream calls, returns the partial answer, and
  reports stopped.
• Manual: 3B model, tools on, ask a multi-step question, tap Stop mid-loop → verify it halts and keeps the partial.

[ASahu16]

Triaged for v1.2.0. Still open on develop — AgentLoop lacks an isCancelled hook; ChatNotifier.stop() only stops the current generation step.