fix: isolate MCP HTTP transports from DefaultTransport in tests (#25821)

Use testing.Testing() inside createTransport to automatically
clone http.DefaultTransport when running in tests. In production,
DefaultTransport is used as-is (efficient connection pooling).

This fixes the CloseIdleConnections flake class: httptest.Server.Close()
calls http.DefaultTransport.CloseIdleConnections(), which disrupts
any MCP client sharing that transport. The testing.Testing() check
means every MCP transport created during tests gets isolation
automatically, with no caller changes needed.

Closes coder/internal#1016
Closes PLAT-291

coderd/x/chatd/mcpclient/mcpclient.go

@@ -285,31 +285,24 @@ func createTransport(
 	cfg database.MCPServerConfig,
 	headers map[string]string,
 ) (transport.Interface, error) {
-	// Each connection gets its own HTTP client with a dedicated
-	// transport so that httptest.Server.Close() (which calls
-	// CloseIdleConnections on http.DefaultTransport) does not
-	// disrupt unrelated connections during parallel tests.
-	var httpClient *http.Client
-	if dt, ok := http.DefaultTransport.(*http.Transport); ok {
-		httpClient = &http.Client{Transport: dt.Clone()}
-	} else {
-		httpClient = &http.Client{}
-	}
+	httpClient := mcpHTTPClient()
 
 	switch cfg.Transport {
 	case "sse":
-		return transport.NewSSE(
-			cfg.Url,
-			transport.WithHeaders(headers),
-			transport.WithHTTPClient(httpClient),
-		)
+		var opts []transport.ClientOption
+		opts = append(opts, transport.WithHeaders(headers))
+		if httpClient != nil {
+			opts = append(opts, transport.WithHTTPClient(httpClient))
+		}
+		return transport.NewSSE(cfg.Url, opts...)
 	case "", "streamable_http":
 		// Default to streamable HTTP, the newer transport.
-		return transport.NewStreamableHTTP(
-			cfg.Url,
-			transport.WithHTTPHeaders(headers),
-			transport.WithHTTPBasicClient(httpClient),
-		)
+		var opts []transport.StreamableHTTPCOption
+		opts = append(opts, transport.WithHTTPHeaders(headers))
+		if httpClient != nil {
+			opts = append(opts, transport.WithHTTPBasicClient(httpClient))
+		}
+		return transport.NewStreamableHTTP(cfg.Url, opts...)
 	default:
 		return nil, xerrors.Errorf(
 			"unsupported transport %q", cfg.Transport,

coderd/x/chatd/mcpclient/mcphttpclient.go

@@ -0,0 +1,25 @@
+package mcpclient
+
+import (
+	"net/http"
+	"testing"
+)
+
+// mcpHTTPClient returns an isolated *http.Client when running
+// inside tests, or nil for production. During tests,
+// httptest.Server.Close() calls
+// http.DefaultTransport.CloseIdleConnections(), which disrupts
+// any MCP client sharing that transport. When DefaultTransport
+// is a *http.Transport it is cloned; otherwise a minimal
+// transport with ProxyFromEnvironment is created as a fallback.
+func mcpHTTPClient() *http.Client {
+	if !testing.Testing() {
+		return nil
+	}
+	if dt, ok := http.DefaultTransport.(*http.Transport); ok {
+		return &http.Client{Transport: dt.Clone()}
+	}
+	return &http.Client{Transport: &http.Transport{
+		Proxy: http.ProxyFromEnvironment,
+	}}
+}