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

agent/x/agentmcp/manager.go

@@ -975,15 +975,19 @@ func (m *Manager) createTransport(ctx context.Context, cfg ServerConfig) (transp
 			}),
 		), nil
 	case "http", "":
-		return transport.NewStreamableHTTP(
-			cfg.URL,
-			transport.WithHTTPHeaders(cfg.Headers),
-		)
+		var opts []transport.StreamableHTTPCOption
+		opts = append(opts, transport.WithHTTPHeaders(cfg.Headers))
+		if c := mcpHTTPClient(); c != nil {
+			opts = append(opts, transport.WithHTTPBasicClient(c))
+		}
+		return transport.NewStreamableHTTP(cfg.URL, opts...)
 	case "sse":
-		return transport.NewSSE(
-			cfg.URL,
-			transport.WithHeaders(cfg.Headers),
-		)
+		var sseOpts []transport.ClientOption
+		sseOpts = append(sseOpts, transport.WithHeaders(cfg.Headers))
+		if c := mcpHTTPClient(); c != nil {
+			sseOpts = append(sseOpts, transport.WithHTTPClient(c))
+		}
+		return transport.NewSSE(cfg.URL, sseOpts...)
 	default:
 		return nil, xerrors.Errorf("unsupported transport %q", cfg.Transport)
 	}

agent/x/agentmcp/mcphttpclient.go

@@ -0,0 +1,25 @@
+package agentmcp
+
+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,
+	}}
+}