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/acp.snapshot.ts

@@ -37,6 +37,7 @@ interface Scenario {
 
 const SCENARIOS: Scenario[] = [
   { name: 'handshake', hasModelTurn: false, recorded: false },
+  { name: 'reject-extra-dirs', hasModelTurn: false, recorded: false },
   { name: 'text-turn', hasModelTurn: true, recorded: true },
   { name: 'tool-call-turn', hasModelTurn: true, recorded: true },
   { name: 'workspace-edit', hasModelTurn: true, recorded: true },