mcp: improve error messages from Wait for streamable clients

Previously, the error message received from ClientSession.Wait would
only report the closeErr, which would often be nil even if the client
transport was broken. Wait should return the reason the session
terminated, if abnormal.

I'm not sure of the exact semantics of this, but surely returning nil is
less useful than returning a meaningful non-nil error. We can refine our
handling of errors once we have more feedback.

Also add a test for client termination on HTTP server shutdown,
described in #265. This should work as long as (1) the session is
stateful (with a hanging GET), or (2) the session is stateless but the
client has a keepalive ping.

Also: don't send DELETE if the session was terminated with 404; +test.

Fixes #265

mcp/streamable_client_test.go

@@ -29,7 +29,7 @@ type streamableRequestKey struct {
 type header map[string]string
 
 type streamableResponse struct {
-	header              header
+	header              header // response headers
 	status              int    // or http.StatusOK
 	body                string // or ""
 	optional            bool   // if set, request need not be sent
@@ -187,6 +187,56 @@ func TestStreamableClientTransportLifecycle(t *testing.T) {
 	}
 }
 
+func TestStreamableClientRedundantDelete(t *testing.T) {
+	ctx := context.Background()
+
+	// The lifecycle test verifies various behavior of the streamable client
+	// initialization:
+	//  - check that it can handle application/json responses
+	//  - check that it sends the negotiated protocol version
+	fake := &fakeStreamableServer{
+		t: t,
+		responses: fakeResponses{
+			{"POST", "", methodInitialize}: {
+				header: header{
+					"Content-Type":  "application/json",
+					sessionIDHeader: "123",
+				},
+				body: jsonBody(t, initResp),
+			},
+			{"POST", "123", notificationInitialized}: {
+				status:              http.StatusAccepted,
+				wantProtocolVersion: latestProtocolVersion,
+			},
+			{"GET", "123", ""}: {
+				status:   http.StatusMethodNotAllowed,
+				optional: true,
+			},
+			{"POST", "123", methodListTools}: {
+				status: http.StatusNotFound,
+			},
+		},
+	}
+
+	httpServer := httptest.NewServer(fake)
+	defer httpServer.Close()
+
+	transport := &StreamableClientTransport{Endpoint: httpServer.URL}
+	client := NewClient(testImpl, nil)
+	session, err := client.Connect(ctx, transport, nil)
+	if err != nil {
+		t.Fatalf("client.Connect() failed: %v", err)
+	}
+	_, err = session.ListTools(ctx, nil)
+	if err == nil {
+		t.Errorf("Listing tools: got nil error, want non-nil")
+	}
+	_ = session.Wait() // must not hang
+	if missing := fake.missingRequests(); len(missing) > 0 {
+		t.Errorf("did not receive expected requests: %v", missing)
+	}
+}
+
 func TestStreamableClientGETHandling(t *testing.T) {
 	ctx := context.Background()