[benbrandt]

Part of the pre-5.0.0 public API cleanup. This PR covers the wire envelopes, the schema dispatch enums, the proxy protocol types, and the derive-to-core contract.

Addressed

• F08: RawJsonRpcMessage::Notification now holds an SDK-owned RawJsonRpcNotification instead of schema::v1::Notification<RawJsonRpcParams>. The wire format does not change.
• F10 / F42: SuccessorMessage is #[non_exhaustive] and gains new(message) and a .meta(..) builder. _meta is typed as Option<serde_json::Map<String, Value>>, and the non-spec meta alias is no longer read. schema::v1::InitializeProxyRequest and schema::v2::InitializeProxyRequest are also #[non_exhaustive].
• F25: Proxies now unwrap incoming proxy/successor params with json_cast_params, so malformed params get -32602 instead of -32700. The duplicate util::internal_error and util::parse_error are removed. The util::json_cast docs now say it is not for inbound params.
• F06: The v1 dispatch enums keep the leading _ on extension methods, as v2 already does. A v1 regression test is added.
• F31: The v1 and v2 dispatch enum macros now delegate method() to the schema enum, so variants the macro doesn't list are sent with their real method name instead of "_unknown".
• F41: MCP_SERVER_UNAVAILABLE and MCP_BACKEND_FAILURE are ErrorCode constants. Adds MCP_RESOURCE_LIMIT_EXCEEDED (-33000).
• G93: Derive output now calls agent_client_protocol::__private::json_cast* instead of public util helpers. The workspace pins the derive crate exactly (=3.6.0). I checked this with release-plz 0.4.2: release-plz set-version rewrites the pin to =5.0.0 and keeps the = operator. Both crates are in the acp version_group, so they always get the same version and the pin stays in sync.
• G94 (partial): The raw envelope types gain Eq. RawJsonRpcMessage gains PartialEq/Eq and From conversions from each envelope.
• G95: The rustdoc now says that unknown request and notification envelope members are dropped.
• F63: Removes util::TypeNotification. Use MatchDispatch::if_notification instead.

Every breaking change has a section in md/migration-v5.md.

Skipped

• G94, ComponentIndex derives: fix(conductor)!: narrow the conductor API and fix multi-version chains #475 (cleanup-conductor) makes ComponentIndex pub(crate) and adds the derives there, so it is left out here to avoid a conflict.

Checks

These were run locally on this head:

• cargo fmt --check
• both clippy runs (-D warnings)
• cargo hack check over the feature powerset at depth 2
• cargo test --workspace --all-features --locked
• cargo test -p agent-client-protocol --no-default-features
• cargo doc with -D warnings for the core, derive, and conductor crates
• mdbook build

All passed except one target. agent-client-protocol-derive --lib aborts at load time (dyld) under the local build-cache runner. The same thing happens on main, so it is not caused by this change.

Review follow-up (3c913a5)

• A proxy/successor whose envelope _meta is not an object now has that _meta ignored, matching how the schema crate treats _meta, instead of failing the forwarded message with -32602.
• Conductor error conversions use Error::into_internal_error.
• The migration guide now says how to compare MCP error codes: error.code == MCP_BACKEND_FAILURE, with .value() where an i32 is needed.
• An independent review also ran a full release-plz update on a scratch clone. It rewrote the derive pin from =3.6.0 to =5.0.0 and moved core and derive to 5.0.0 together.