[benbrandt]

Follow-ups to the schema 1.13 update (#443, #444) that bring Testy, the examples, and the docs in line with the remaining v2 spec changes. #450 already covered the prompt-content rejection, simple_agent_v2's resume session info, and most of the Schema 1.13 section, so this branch adds only what that left out.

Each commit can be reviewed on its own:

• Testy and simple_agent_v2 reject session/list cursors. Neither issues a nextCursor, so any cursor a client sends is invalid and gets -32602, as v1 Testy already does.
• Testy v2 sends session info before its resume response. The session-setup page says agents SHOULD send current session info after any replay and before responding. md/testy.md documents this, the prompt-content and cursor rules, and that the prompt response carries messageId.
• Docs for the acp MCP declaration and remaining 1.13 notes. The v2 with_mcp_server builders declare an McpServer::Acp server, which a conforming agent rejects with -32602 unless it advertises session.mcp.acp. The rustdoc and the protocol v2 chapter now say so and point to McpOverAcpPolyfill. No runtime capability check is added. The Schema 1.13 section also lists the new _meta fields, and the v1 downgrade notes that terminal auth is kept.
• Close ordering and cancelled responses. V2Session::close rustdoc now describes the close ordering from the spec. The cancellation docs no longer suggest answering a cancelled request with partial data: a successful response is allowed only where the method defines one, and -32800 otherwise. The error-handling docs note the reserved code ranges.
• Example clients.
  • v2_one_shot_client and the cookbook's v2 recipe decline permission requests with the agent's reject_once option. They fall back to session/cancel plus the cancelled outcome only when there is no such option.
  • Both treat any non-idle state, including an unknown one, as active work.
  • yopo and v2_one_shot_client strip control characters from agent text before printing it, per the security considerations.
• Dead downgrade keys. The v1 initialize downgrade no longer lists nes and positionEncodings, which schema 1.12 removed.

Testing:

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features --locked
• cargo test --workspace --all-features --locked --exclude agent-client-protocol-derive: 951 passed

The derive crate's tests were excluded because their test binary fails to load locally (a dyld rpath issue in the local runner); none of these commits touch that crate.