test(acp-example): snapshot the session/new workspace-scope rejection

Master's "fix(acp): align prompt and workspace contracts" made session/new
reject a non-empty additionalDirectories / mcpServers (widening the workspace
scope is unimplemented). Add a `reject-extra-dirs` scenario + a
`newSessionExpectError` input op that pins this editor-facing contract: the
bridge answers with `-32602 Invalid params: additionalDirectories is not
supported`. Keyless, deterministic, no model call.

(session/load replay — the other new master behavior — needs a two-phase
seed-then-load harness and is left for a focused follow-up.)

examples/acp-agent/tests/snapshots/reject-extra-dirs/input.json

@@ -0,0 +1,6 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSessionExpectError", "additionalDirectories": ["/extra-dir"] }
+  ]
+}

examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl

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

examples/acp-agent/tests/snapshots/reject-extra-dirs/stdout.golden.jsonl

@@ -0,0 +1,2 @@
+{"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,"error":{"code":-32602,"message":"Invalid params: additionalDirectories is not supported in this MVP"}}