* feat(transport): add Unix domain socket client for streamable HTTP
MCP hosts in Kubernetes environments with Envoy sidecars need to route
HTTP through Unix domain sockets because DNS-based URIs only resolve
via the proxy. Adds UnixSocketHttpClient implementing StreamableHttpClient
using hyper over tokio::net::UnixStream, gated behind the
transport-streamable-http-client-unix-socket feature.
Also extracts RESERVED_HEADERS, extract_scope_from_header, and
validate_custom_header into common/http_header.rs to share header
validation logic between the reqwest and unix socket implementations.
* fix(transport): address review feedback for unix socket transport
- Document one-connection-per-request behavior on UnixSocketHttpClient
- Reject empty socket paths and bare '@' in constructor with assert
- Add explicit dep:http to unix-socket feature for self-documenting deps
- Document MCP-Protocol-Version exception on RESERVED_HEADERS constant
- Fix test catch-all to echo request id instead of hardcoding 1
- Remove leftover sleep(100ms) in test_unix_socket_custom_headers
- Add blank line before macro comment in Cargo.toml
* fix(transport): fix CI failures for unix socket transport
- Use std::io::Error::other() instead of Error::new(ErrorKind::Other)
to satisfy clippy::io_other_error on newer nightly
- Use #[tokio::test(flavor = "current_thread")] for unix socket tests
since axum's serve(UnixListener) requires spawn_local
- Gate validate_custom_header behind client-side-sse feature since it
references http::HeaderName which isn't available with default features
* fix(transport): fix CI failures for unix socket transport
axum::serve(UnixListener) uses spawn_local on Linux, which panics
outside a LocalSet. Replace with manual hyper HTTP/1.1 server that
accepts connections directly from the UnixListener, avoiding the
spawn_local requirement entirely.
* fix(transport): skip unix socket tests when local feature is enabled
The local feature causes ().serve(transport) to use spawn_local, which
requires a LocalSet. Gate the integration tests with not(feature = "local")
to match every other integration test in the repo.
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