test(transport): register streamable-http disconnect-cancel test (#857)

Add the `[[test]]` entry so the new integration test is only built when
its required features (server, transport-streamable-http-server, reqwest)
are enabled, matching the other streamable-http test targets.

Author: ameyypawar

crates/rmcp/Cargo.toml

@@ -387,3 +387,12 @@ required-features = [
   "transport-streamable-http-client-reqwest",
 ]
 path = "tests/test_streamable_http_connection_reuse.rs"
+
+[[test]]
+name = "test_streamable_http_disconnect_cancel"
+required-features = [
+  "server",
+  "transport-streamable-http-server",
+  "reqwest",
+]
+path = "tests/test_streamable_http_disconnect_cancel.rs"