[benbrandt]

Part of the 5.0.0 public-API cleanup. This PR covers the HTTP crate, mcp_server, the rmcp crate, AcpAgent, and some cookbook/docs nits. Breaking changes are listed in md/migration-v5.md.

Addressed

• F11 HttpClientError: now #[non_exhaustive] with rustdoc. Its causes use #[source] instead of #[from], and Display no longer repeats the source.
• F76 CorsOptions: now an opaque struct built through its constructors. The Vec<HeaderValue> payload is no longer public. md/migration-config-api.md describes the 3.x enum and the v5 struct separately.
• F43 The HTTP crate re-exports axum (server) and reqwest/url (client).
• F14 Crate docs for the HTTP crate, plus AcpHttpServer rustdoc with a v1+v2 AgentProtocolRouter example and a # Panics section on into_router (covered by tests).
• F17 EnabledTools is crate-private.
• F77 enable_tool/disable_tool return a local UnknownToolError instead of a -32600 Error. ? still converts it.
• F78 McpServerExt is sealed.
• F47 McpConnectionContext::Acp is #[non_exhaustive].
• F36 Builder-built servers report the builder name and a new McpServerBuilder::version (default 0.0.0) as serverInfo. The trace snapshot is updated.
• F37 Builder tool errors, including bad arguments, become isError: true results. Unknown or disabled tools still get -32602. RegisteredMcpTool::call_tool reports bad arguments as -32602 instead of -32603, covered by a unit test.
• F35 rmcp now picks the native MCP-over-ACP path from the core crate's feature, using a hidden __if_mcp_over_acp! macro. Its rustdoc says it is a stable cross-crate contract for 5.x, so renaming it is breaking. Fully un-gating the constructor isn't possible because McpService uses schema types that are only available behind unstable_mcp_over_acp.
• F24 Dropped rmcp's no-op unstable_protocol_v2 feature. The v2 example gets the core feature through a dev-dependency.
• F55 The tool_fn/tool_fn_mut builder examples compile, including the macro argument.
• F09 AcpAgent::spawn_process is pub(crate).
• F61 Removed the AcpAgent::claude_agent()/codex() presets, which ran unpinned @latest npx packages. The migration guide shows from_str with a pinned version.
• F28 MatchDispatch::otherwise_ignore now answers unmatched requests with -32601. The examples use method_not_found(). Fixed the stale "no effect" and handle_if docs. The migration guide covers the change and its effect on v1::ActiveSession::read_to_string, where requests routed to the session that it does not handle now get -32601.
• F56 The v1 permission recipe sends session/cancel before answering cancelled. It now compiles as a doctest.
• F54 The custom handler recipe now says it parses a raw Dispatch as v1 types.
• G84 The WebAssembly docs explain the Send requirement and how to keep !Send host values outside the connection.
• Flaky test graceful_shutdown_sigkills_child_ignoring_sigterm: the scripts install the TERM trap before writing the PID file that starts shutdown, and the terminate window is now 1s (the test takes about 1.1s). 16 parallel runs under extra CPU load all passed.

Skipped

• F66 (HTTP client constructor naming): the names already pair up consistently (new/builder for a base URL, with_endpoint/builder_with_endpoint for an exact URL, from_http_client for a prebuilt client). Renaming them would churn the docs and callers for little gain.