Two rmcp tests fail on Windows: connection-refused text check and npm spawn in test_with_js #1364

Describe the bug
I ran rmcp's tests on Windows (CI only runs Linux). Two tests fail there. Both problems are in the tests; the SDK code behaves correctly.

1. transport::auth::tests::default_http_client_preserves_connection_failure_cause (lib, --all-features) checks for the English text connection refused. Windows describes a refused connection differently, so the assertion fails although the error chain is correct:

   unexpected discovery error: Metadata error: OAuth metadata discovery failed for http://127.0.0.1:34350/mcp
     Caused by: error sending request for url (http://127.0.0.1:34350/mcp)
     Caused by: client error (Connect)
     Caused by: tcp connect error
     Caused by: No connection could be made because the target machine actively refused it. (os error 10061)
   

2. All three test_with_js tests (no-local feature set) fail with Error: program not found. install_js_dependencies runs Command::new("npm"), but on Windows npm is the shim npm.cmd, and Command doesn't resolve a bare name to a .cmd file. (node is node.exe, so the other spawns work.)

To Reproduce
On Windows 11 with rustc 1.96 (the version rust-toolchain.toml pins), Node 24 and uv:

1. cargo test -p rmcp --all-features --lib default_http_client_preserves_connection_failure_cause
2. cargo test -p rmcp --features "<all rmcp features except local, as in CI>" --test test_with_js

Expected behavior
Both pass, as they do on Linux.

Proposed fix (test-only, 8 lines added, 3 removed)

• auth test: before the request, connect to the same dropped address with std::net::TcpStream::connect(addr), and assert that the reason contains that error's to_string(). That way the test expects whatever text the OS uses, instead of a hard-coded English phrase.
• test_with_js: let npm = if cfg!(windows) { "npm.cmd" } else { "npm" };. Another option is to dogfood rmcp::transport::which_command("npm"), but then which-command has to be added to the test's required-features.

Patch: main...RafalOfficial:rust-sdk:fix/windows-test-portability. With it, both tests pass on Windows. These all pass unchanged on Windows: the other 606 lib tests, test_with_python, test_stdio_response_concurrency, test_close_connection and test_async_rw_max_line_length. The patch is formatted with cargo +nightly fmt. If you're happy with this, I can open the PR. If you don't want Windows-only test fixes while CI is Linux-only, just close this.

Additional context
Main at 8d226f4. This report and the proposed patch were written with Claude Code (AI-assisted). I ran the commands above on a real Windows machine.