test(acp-example): five snapshot scenarios + cancel/error input ops

Adds the first cut of snapshot scenarios, each asserting a normalized stdout
transcript golden and (for model turns) a re-persisted session-log golden:

- text-turn, tool-call-turn, multi-turn: RECORDED against the real API — the
  committed session.jsonl is a genuine harvested log; replay derives the model
  script from it and reproduces deterministically with no key. tool-call-turn
  exercises the real bash executor (echo SNAPSHOT_OK → tool/call + tool/result
  + a post-tool answer step).
- error-finish, cancel: AUTHORED via a replay.override.json sidecar (the live
  API can't be coaxed into a deterministic 401 or mid-stream cancel). error-
  finish replays a {kind:throw} 401 → the bridge answers the prompt with a
  JSON-RPC error and the log records turn/end{kind:error}; cancel replays a
  {kind:hang} → stopReason:cancelled.

Two input-DSL ops support these: promptExpectError (awaits the prompt, asserts
it rejects — the editor's view of a failed turn — and swallows it) and
promptAndCancel (dispatches the prompt unawaited, waits until the client
OBSERVES the streamed agent_message_chunk, then cancels — pinning frame order
so the cancel transcript is deterministic; fixes a flake Codex caught where the
late chunk and the cancelled response could interleave either way).

Scenarios carry a `recorded` flag so test:snapshot:record only re-runs the
live-API ones. reasoning/max-tokens scenarios are deferred (hard to force
deterministically from the live model). Per docs/rfc/implemented/2026-06-19.

examples/acp-agent/tests/snapshots/cancel/input.json

@@ -0,0 +1,7 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSession" },
+    { "op": "promptAndCancel", "text": "Start a long task; this turn will be cancelled mid-stream." }
+  ]
+}

examples/acp-agent/tests/snapshots/cancel/replay.override.json

@@ -0,0 +1,3 @@
+[
+  { "kind": "hang" }
+]

examples/acp-agent/tests/snapshots/cancel/session.golden.txt

@@ -0,0 +1,95 @@
+{
+  "type": "session",
+  "version": 1,
+  "id": "{{sessionId}}",
+  "createdAt": 0,
+  "cwd": "{{cwd}}"
+}
+{
+  "type": "turn/start",
+  "seq": 0,
+  "time": 0,
+  "data": {
+    "turn": 1,
+    "trigger": {
+      "kind": "message",
+      "source": {
+        "kind": "user"
+      }
+    }
+  }
+}
+{
+  "type": "user/message",
+  "seq": 1,
+  "time": 0,
+  "data": {
+    "content": [
+      {
+        "type": "text",
+        "text": "Start a long task; this turn will be cancelled mid-stream."
+      }
+    ],
+    "source": {
+      "kind": "user"
+    }
+  }
+}
+{
+  "type": "step/start",
+  "seq": 2,
+  "time": 0,
+  "data": {
+    "turn": 1,
+    "step": 1
+  }
+}
+{
+  "type": "assistant/chunk",
+  "seq": 3,
+  "time": 0,
+  "data": {
+    "turn": 1,
+    "step": 1,
+    "chunk": {
+      "type": "block-start",
+      "index": 0,
+      "blockType": "text"
+    }
+  }
+}
+{
+  "type": "assistant/chunk",
+  "seq": 4,
+  "time": 0,
+  "data": {
+    "turn": 1,
+    "step": 1,
+    "chunk": {
+      "type": "text-delta",
+      "index": 0,
+      "text": "partial"
+    }
+  }
+}
+{
+  "type": "step/end",
+  "seq": 5,
+  "time": 0,
+  "data": {
+    "turn": 1,
+    "step": 1
+  }
+}
+{
+  "type": "turn/end",
+  "seq": 6,
+  "time": 0,
+  "data": {
+    "turn": 1,
+    "reason": {
+      "kind": "aborted",
+      "reason": "session/cancel"
+    }
+  }
+}

examples/acp-agent/tests/snapshots/cancel/session.jsonl

@@ -0,0 +1 @@
+{"type":"session","version":1,"id":"00000000-0000-0000-0000-000000000000","createdAt":0}

examples/acp-agent/tests/snapshots/cancel/stdout.golden.txt

@@ -0,0 +1,62 @@
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "result": {
+    "protocolVersion": 1,
+    "agentInfo": {
+      "name": "deepseek-harness-acp",
+      "version": "0.0.1"
+    },
+    "agentCapabilities": {
+      "loadSession": true,
+      "promptCapabilities": {
+        "image": false,
+        "audio": false,
+        "embeddedContext": false
+      }
+    },
+    "authMethods": []
+  }
+}
+{
+  "jsonrpc": "2.0",
+  "id": 2,
+  "result": {
+    "sessionId": "{{sessionId}}"
+  }
+}
+{
+  "jsonrpc": "2.0",
+  "method": "session/update",
+  "params": {
+    "sessionId": "{{sessionId}}",
+    "update": {
+      "sessionUpdate": "user_message_chunk",
+      "content": {
+        "type": "text",
+        "text": "Start a long task; this turn will be cancelled mid-stream."
+      }
+    }
+  }
+}
+{
+  "jsonrpc": "2.0",
+  "method": "session/update",
+  "params": {
+    "sessionId": "{{sessionId}}",
+    "update": {
+      "sessionUpdate": "agent_message_chunk",
+      "content": {
+        "type": "text",
+        "text": "partial"
+      }
+    }
+  }
+}
+{
+  "jsonrpc": "2.0",
+  "id": 3,
+  "result": {
+    "stopReason": "cancelled"
+  }
+}