fix(http): disable idle connection pooling for default client

POST SSE response bodies cannot be fully consumed before the next
request starts, causing reqwest to stall on pool checkout (~40ms).
The long-lived GET SSE stream already holds its own connection, so
there is no benefit to pooling POST connections with HTTP/1.1.

This matches the workaround reported in #789.

Author: DaleSeo

crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs

@@ -262,7 +262,7 @@ impl StreamableHttpClientTransport<reqwest::Client> {
     /// This method requires the `transport-streamable-http-client-reqwest` feature.
     pub fn from_uri(uri: impl Into<Arc<str>>) -> Self {
         StreamableHttpClientTransport::with_client(
-            reqwest::Client::default(),
+            Self::default_http_client(),
             StreamableHttpClientTransportConfig {
                 uri: uri.into(),
                 auth_header: None,
@@ -277,7 +277,21 @@ impl StreamableHttpClientTransport<reqwest::Client> {
     ///
     /// * `config` - The config to use with this transport
     pub fn from_config(config: StreamableHttpClientTransportConfig) -> Self {
-        StreamableHttpClientTransport::with_client(reqwest::Client::default(), config)
+        StreamableHttpClientTransport::with_client(Self::default_http_client(), config)
+    }
+
+    /// Build the default reqwest client for this transport.
+    ///
+    /// Disables idle connection pooling because POST SSE response bodies
+    /// cannot be fully consumed before the next request starts, causing
+    /// reqwest to stall on pool checkout (~40 ms).  The long-lived GET
+    /// SSE stream already holds its own connection, so there is no
+    /// benefit to pooling POST connections with HTTP/1.1.
+    fn default_http_client() -> reqwest::Client {
+        reqwest::Client::builder()
+            .pool_max_idle_per_host(0)
+            .build()
+            .expect("failed to build default reqwest client")
     }
 }