fix(acp): preserve interrupted responses

[danielkov]

Summary

Preserve partial ACP v2 assistant responses when users interrupt a turn, and retain that partial response in model context for the next turn.

Motivation

Interrupted turns currently replace streamed output with empty content and omit the partial assistant response from subsequent model context. This leaves a blank, unfinished UI entry and breaks conversational continuity.

Impact

Interrupted ACP v2 turns keep the response content streamed before cancellation. Retry attempts that are explicitly superseded continue to clear stale output before rendering their replacement.

Technical details

Distinguish cancellation from supersession

Only ResponseAttemptSuperseded clears provisional ACP messages. Ordinary cancellation preserves the current message and thought chunks.

Preserve interrupted model context

The AgentKit revision records streamed partial assistant content in the cancelled turn result and transcript, resets it on supersession, and processes supersession before concurrent cancellation.

Track the maintained AgentKit branch

Both agentkit-acp and agentkit-loop are pinned to commit 6480d90 on the maintained feat/acp-v2-session-inject branch so the ACP and loop implementations resolve from one durable source revision.

[kit-code-agent]

The ACP cancellation behavior is internally consistent and its targeted tests pass. Two issues in the newly selected AgentKit loop source should be addressed before merging.

[kit-code-agent]

Re-review of the latest head found no new issues. Both prior findings remain valid: the refreshed AgentKit pin does not bound the cancellation-only streamed-content accumulator, and although the new commit is now reachable from feat/acp-v2-session-inject, it remains unsigned post-release code identified as agentkit-loop 0.10.11 rather than a distinct checksummed release.

[kit-code-agent]

Re-review of the latest head found no new issues. The interrupted-response buffer finding is addressed by AgentKit commit 7604f6c, which caps retention at 1 MiB and 256 parts, releases buffered content on overflow, and resets the budget on supersession. That prior thread is resolved. The remaining release-integrity finding is still valid and remains unresolved because the selected code is still unsigned, non-checksummed git source labeled agentkit-loop 0.10.11.

[kit-code-agent]

No issues found. The changes look good to merge.