Attempt to fix flaky handover test (#3822)

## Changes
Use fewer messages, increase waiting for writes timeouts, and add
don't-buffer-output flag to the `cat` command for the test server.

## Why
Failed handover test run:
https://github.com/databricks/cli/actions/runs/18774560101/job/53566106289

Author: ilia-db

experimental/ssh/internal/proxy/client_server_test.go

@@ -25,7 +25,8 @@ func createTestServer(t *testing.T, maxClients int, shutdownDelay time.Duration)
 	connections := NewConnectionsManager(maxClients, shutdownDelay)
 	proxyServer := NewProxyServer(ctx, connections, func(ctx context.Context) *exec.Cmd {
 		// 'cat' command reads each line from stdin and sends it to stdout, so we can test end-to-end proxying.
-		return exec.CommandContext(ctx, "cat")
+		// '-u' option is used to disable output buffering.
+		return exec.CommandContext(ctx, "cat", "-u")
 	})
 	return httptest.NewServer(proxyServer)
 }

experimental/ssh/internal/proxy/proxy_test.go

@@ -25,8 +25,8 @@ type testBuffer struct {
 }
 
 const (
-	MESSAGE_CHUNKS      = 5
-	MESSAGES_PER_CHUNK  = 1024
+	MESSAGE_CHUNKS      = 4
+	MESSAGES_PER_CHUNK  = 512
 	TOTAL_MESSAGE_COUNT = MESSAGE_CHUNKS * MESSAGES_PER_CHUNK
 )
 
@@ -61,7 +61,7 @@ func (tb *testBuffer) AssertWrite(expected []byte) error {
 	case data := <-tb.OnWrite:
 		assert.Equal(tb.t, expected, data)
 		return nil
-	case <-time.After(2 * time.Second):
+	case <-time.After(3 * time.Second):
 		return errors.New("timeout waiting for write, was expecting: " + string(expected))
 	}
 }
@@ -79,7 +79,7 @@ func (tb *testBuffer) WaitForWrite(expected []byte) error {
 			if tb.Contains(expected) {
 				return nil
 			}
-		case <-time.After(2 * time.Second):
+		case <-time.After(3 * time.Second):
 			return errors.New("timeout waiting for write")
 		}
 	}