feat(acp): stream session/prompt deltas as session/update chunks (#3192)

[findshan]

What & why

Follow-up to #3698 (concurrent session/cancel). The ACP adapter still buffered the whole turn: it called the non-streaming create_message and emitted a single session/update chunk only after the provider finished. Editors like Zed render agent output incrementally, so a user saw nothing until the entire response landed — and the readiness audit in docs/ACP_REGISTRY_SUBMISSION.md explicitly listed non-streaming as a known limitation. This closes that gap.

Approach

Stream the turn, reusing the existing LlmClient::create_message_stream path the TUI engine already uses (turn_loop.rs), so no new provider plumbing:

• open_prompt_stream builds the request with stream: true and returns the 'static StreamEventBox. It borrows &self only to read config/model and open the stream, so the caller can race it against the reader without holding a server borrow. (Replaces the old run_prompt that awaited a full create_message.)
• drive_prompt_stream consumes the stream concurrently with the input reader:
  • each text delta (content_block_delta / text content_block_start) → its own session/update agent_message_chunk, emitted as it arrives;
  • message_stop (or stream end) → PromptOutcome::Completed(full_text), recorded in session history;
  • a provider stream error → JSON-RPC -32603.
• Cancellation preserved, now mid-stream: a matching session/cancel returns stopReason: "cancelled" and dropping the stream aborts the underlying provider connection. Single-flight semantics unchanged (other-session cancel ignored; concurrent request rejected with a clear error). The single-writer invariant holds, so streamed chunks + acks all stay on one protocol-clean stdout stream.

Testing

The streaming + cancellation control logic stays a free function over the reader/writer + boxed stream, so it's unit-tested deterministically with canned in-memory streams — no real provider. 4 tests:

• drive_prompt_streams_each_delta_as_a_chunk_then_completes — two deltas → two session/update chunks, full text accumulated.
• drive_prompt_cancels_when_matching_cancel_arrives — cancel wins over a pending stream → Cancelled.
• drive_prompt_ignores_cancel_for_a_different_session — other-session cancel acked + ignored, turn completes.
• drive_prompt_rejects_a_concurrent_request_but_keeps_running — concurrent request gets -32603, turn completes.

Local: cargo fmt --all -- --check clean; clippy (bin, repo flags) clean; full bin suite 5521 passed, 0 failed.

Scope

• Streaming covers text deltas only; thinking/tool/server-tool deltas aren't surfaced over ACP (baseline here is text-only, tools: None). Doc updated to say so.
• Stacks on feat(acp): cancel in-flight session/prompt on session/cancel (#3192) #3698 — it merged to main, so this is a clean diff against current main.

Refs #3192.

[github-actions]

Thanks @findshan for taking the time to contribute.

This repository is observing a maintainer-managed PR intake gate in dry-run mode, so this pull request is staying open. This note helps maintainers prepare the allowlist before any enforcement is considered.

Please read CONTRIBUTING.md for the expected contribution shape. A maintainer can grant recurring PR access by commenting /lgtm on a pull request.