fix(test): gate cancelled response tests on stdio features

[LucaCiucci]

Declare server and transport-io as required features for test_cancelled_response.

Motivation and Context

The test calls transport::stdio(), but transport-io is not enabled by default. Without the feature requirement, default-feature test builds fail to compile. Cargo now skips this test when its required features are unavailable.

How Has This Been Tested?

• A bare cargo test does not fail anymore.
• Confirmed Cargo reports the required features when explicitly selecting the test without transport-io.
• Ran cargo test -p rmcp --test test_cancelled_response --features transport-io: both tests pass.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• (N/A) I have added appropriate error handling
• (N/A) I have added or updated documentation as needed

Additional context

N/A

[github-actions]

@bolinfest is on triage rotation this week and is the reviewer for this one.

Labels come from the PR labeler. Please review the change.

_{Automated triage. Labels are a first pass and the assigned maintainer has the final say.}

[LucaCiucci]

I think the automated labels are wrong. T-dependencies doesn't sound right to me.

[DaleSeo]

Suggested change

	[[test]]
	name = "test_cancelled_response"
	path = "tests/test_cancelled_response.rs"
	required-features = ["server", "transport-io"]
	[[test]]
	name = "test_cancelled_response"
	required-features = ["server"]
	path = "tests/test_cancelled_response.rs"

Gating the entire target on transport-io also skips the three tests here gated on client. They only use duplex and mpsc, and never touch stdio.

// tests/test_cancelled_response.rs
+#[cfg(feature = "transport-io")]
 #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
 async fn cancelled_request_receives_no_response() -> anyhow::Result<()> {
 ...
+#[cfg(feature = "transport-io")]
 #[tokio::test]
 // the HELPER_ENV entry point, serve_helper_stdio, spawn_helper

[DaleSeo]

I think the automated labels are wrong. T-dependencies doesn't sound right to me.

#1368 will fix that