[benbrandt]

Part of the 5.0 public-API cleanup. Tightens the version-neutral root connection and builder surface so that adding later protocol versions stays additive.

Addressed

• F01 Remove Builder::with_v2_protocol_guard (all four impls). The 2.1.0 changelog entry is left as history; the migration guide records the removal.
• F15 Merge ConnectionTo::spawn_connection_with_context into spawn_connection, which now returns P::Connection<R::Counterpart> (a raw builder still returns the raw ConnectionTo).
• F18 ConnectionTo::into_protocol::<P>() returns Result and fails with an Internal error (-32603) when the builder's guard selected another version. The mismatch is a local programming error, so it is not reported to the peer as Invalid Request. Adds ConnectionTo::protocol_version().
• F04 Typed send_cancel_request(_to) take impl Into<schema::v1::RequestId> / impl Into<schema::v2::RequestId>. v1 stays source-compatible because RawJsonRpcId converts into schema::v1::RequestId. Only v2 callers passing a RawJsonRpcId must switch.
• G82 From<v1::ConnectionTo<C>> and From<v2::ConnectionTo<C>> for ConnectionTo<C>, and Protocol::Connection requires Into<ConnectionTo<C>>.
• F20 InitializationVersionMismatch and SelectedInitializeResponse move to the crate root (still behind unstable_protocol_v2). InitializeOutcome stays in v2::.
• F75 Drop the redundant InitializationVersionMismatch::selected field. Use the selected() method instead.
• F27 ProxyProtocolRouter shares AgentProtocolRouter's answer-and-continue selection loop. It no longer closes the connection on the first rejected or malformed frame. Adds a retry test and a test for a v1+v2 router that answers invalid JSON and a batch with id: null errors, then forwards the next valid proxy/initialize to the selected implementation unchanged. The rejection-drain tests in protocol_driver_finish.rs are replaced by one that checks the rejection is flushed over Lines while input stays open and that the router finishes on input EOF.
• F38 Channel is #[non_exhaustive], with Channel::new and Channel::into_parts. The rustdoc now states that the unbounded mpsc pair is the contract.
• F39 JsonRpcMessage::parse_message takes params: &serde_json::Value. This changes the signature only, and the derive macro output is updated to match.
• F50 SentRequest and PreparedRequest gain into_raw and, on Raw, into_protocol.

Skipped

• F69 SelectedInitializeRequest is defined in the conductor crate, not in core. It belongs to the conductor cleanup.

Notes

• The http crate and core tests needed small edits: .. in Channel patterns and the new parse_message callers.
• The derive crate output changed for F39, so core and derive have to be released together.
• session.rs's ensure_v1_session_protocol is now effectively unreachable, because into_protocol::<V1>() rejects v2 connections. I left it alone because fix(acp)!: v1 session stream reports prompt errors and full responses #474 rewrites much of that file. It should be removed or turned into a debug_assert! there or in a follow-up.
• With F27, a proxy router that has v1 configured now answers a malformed frame (invalid JSON or a batch) before initialization with an error, as AgentProtocolRouter does. It used to hand that frame to the v1 implementation.

Checks

Rerun at 3791556:

• cargo fmt -- --check: pass
• cargo clippy --workspace --all-targets --all-features -- -D warnings and cargo clippy -- -D warnings: pass
• cargo hack check --workspace --feature-powerset --depth 2 --exclude-features default,unstable,wasm_js --no-dev-deps: pass, no warnings
• cargo test --workspace --all-features --locked --no-fail-fast: pass, except agent-client-protocol-derive --lib, which fails to load libstd under the local test runner (it does on main too)
• cargo test -p agent-client-protocol --no-default-features --locked: pass
• RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps for core, derive, http and test crates: pass
• mdbook build: pass