David Solomon, wearing the full academic scholarship, joyfully shook his head and moved his index finger in the rhythm of his own AI-generated music. Goldman Sachs' boss — who in May touched the bottom line of a motivational anthem for MBA graduates — may not be the only Wall Street executive to dance. With a start of 2026 that broke all records, the biggest banks in the U.S. are rediscovering their self-confidence and prestige. Goldman's action…