[benbrandt]

Implements the reference-conductor differences listed in agentclientprotocol/agent-client-protocol#2743 (proxy chains RFD cleanup), plus the v2 default error for tool_call/cancel and terminal/cancel. It was stacked on #480 and is now rebased onto main, including #481 (v1::schema / v2::schema paths). No public API changes.

Changes

1. The conductor rejects misplaced proxy/successor (fix(conductor))

proxy/successor is exchanged only between the conductor and the proxies it initialized, and between a conductor in proxy mode and its own conductor. The conductor used to forward a proxy/successor from its client (agent mode) to the first proxy, which treated it as coming from its successor, and one from the agent to the last proxy double-wrapped. Either side could forge messages that appear to come from the other.

The conductor now answers such a request with -32600 and drops and logs such a notification. The same applies to a proxy/successor wrapped twice, by a proxy or by the outer conductor of a conductor in proxy mode.

Tests (conductor/tests/proxy_wire.rs): agent mode (from the client, from the agent, double-wrapped from a proxy), proxy mode (double-wrapped from the outer conductor), and regressions showing legitimate proxy -> conductor and outer conductor -> proxy proxy/successor traffic still flows.

2. Proxies send $/cancel_request plain (fix(acp))

A proxy cancelling a request it sent to its successor used to wrap $/cancel_request in proxy/successor. Per #2743 it now sends it plain, with the ID of the proxy/successor request on that hop. SentRequest::cancel, RequestCancellationHandle::cancel and ConnectionTo::send_cancel_request_to always send it plain. Receivers, including the conductor, still accept the wrapped form, so a v5 conductor works with older proxies.

Tests: a two-proxy chain (v1 and v2) with tapped proxy connections asserts that every hop carries exactly one plain $/cancel_request naming that hop's request, and that the agent's request is cancelled. Another test checks that the conductor maps both a plain and a wrapped cancel from a proxy to the forwarded request, in agent mode and proxy mode. The existing core tests that expected the wrapped form now expect the plain one.

3. v2 default -32602 for tool_call/cancel and terminal/cancel (fix(acp))

An initialized Agent.v2() agent without a handler for these requests answers -32602 ("the tool call is unknown or not cancellable", with {toolCallId} / {terminalId} in data), matching the spec and the TypeScript SDK. A typed or catch-all handler takes precedence. Before initialization the usual -32600 applies. v1 and unguarded (Builder::new) agents still answer -32601. Testy v2 and simple_agent_v2 pick this up through the default.

Tests: unhandled -> -32602, plus -32600 before initialization; typed and catch-all handlers win; v1 (Testy) and raw agents still answer -32601; Testy v2 answers -32602.

Wire-format changes

• $/cancel_request from a proxy to its conductor is now plain instead of wrapped in proxy/successor. The wrapped form is still accepted.
• The conductor answers proxy/successor from the client (agent mode) or the agent with -32600 instead of forwarding it.
• v2 agents answer unhandled tool_call/cancel / terminal/cancel with -32602 instead of -32601.

Docs: md/conductor.md, md/protocol.md, md/request-cancellation.md, md/protocol-v2.md, the cancellation rustdoc chapter, and migration notes in md/migration-v5.md.