Make the fixture-server readiness check detect refused connections

[jordan-brough]

What you'll see

Run one of the SSE spec files on its own on a Mac, for example:

bundle exec rspec spec/ruby_llm/mcp/native/transports/sse_spec.rb

Every example fails with SSE transport closed. The streamable spec file fails one example with Connection refused. The full suite passes, because earlier spec files take long enough for the fixture servers to finish booting. So this is easy to miss.

Why it happens

TestServerManager starts the fixture servers and then asks "is the port open yet?" before letting the tests run. That check uses Socket.tcp(host, port, connect_timeout: 1).

On macOS with Ruby up to 4.0.6, that call answers "yes" even when nothing is listening on the port. This is a Ruby bug: https://bugs.ruby-lang.org/issues/22223. The retried connect(2) reports EISCONN and Ruby did not check SO_ERROR, so a refused connection looked like a successful one.

As a result, the readiness check returns right away and the tests start before the servers are up. The first spec to talk to a server gets a refused connection.

The fix

Use TCPSocket.new(host, port, connect_timeout: 1) for the probe instead. Its connect path checks SO_ERROR and raises ECONNREFUSED correctly, so start_server blocks until every fixture port is really accepting connections.

One file, a few lines: spec/support/test_server_manager.rb (wait_for_port and port_open?), plus a short comment explaining why.

What changes in behavior

Server startup now waits until the ports are really listening, which takes about half a second. The isolated runs pass. The full suite is unchanged.

Measured on macOS with Ruby 4.0.6:

	Before	After
start_server returns after	0.006 to 0.008 s, with none of ports 3005/3006/3007 listening (they came up about 0.37 s later)	0.63 to 0.84 s, with all three ports listening
native/transports/sse_spec.rb alone	48 examples, 48 failures	48 examples, 0 failures
adapters/mcp_sdk_sse_spec.rb alone	6 examples, 6 failures	6 examples, 0 failures
adapters/mcp_sdk_streamable_spec.rb alone	6 examples, 1 failure	6 examples, 0 failures

Ruby 4.0.7 fixes the underlying Socket.tcp bug, but the probe should be correct on older Rubies too.

[patvice]

Sweet! Thanks for this @jordan-brough. Merging this now