Fix flaky TestHandover by increasing timeout to 500ms (#3749)

## Changes

The 10ms handover timeout was too aggressive for the connection handover
process, which involves creating new WebSocket connections, closing old
ones, and waiting for acknowledgments. Changed to 500ms to allow
reliable completion.

## Why

Failure:
https://github.com/databricks/cli/actions/runs/18462066531/job/52595573781

Author: pietern

experimental/ssh/internal/proxy/client_server_test.go

@@ -144,7 +144,7 @@ func TestHandover(t *testing.T) {
 	defer server.Close()
 
 	maxHandoverCount := 3
-	handoverTimeout := 10 * time.Millisecond
+	handoverTimeout := 500 * time.Millisecond
 	createConnChan := make(chan error, 1)
 	clientInputWriter, clientOutput := createTestClient(t, server.URL, handoverTimeout, nil, createConnChan)
 	defer clientInputWriter.Close()