When investment-banking giant Goldman Sachs bought 11 Seattle and Eastside office buildings and complexes in 2007 — overnight becoming one of the market’s largest landlords — there wasn’t much talk of risk. The 2.5-million-square-foot portfolio was almost fully leased, its market value on the rise. Stable, venerable Washington Mutual, the largest tenant, had been locked in for another decade. A Goldman affiliate, Whitehall Street Global Real Estate Limited Partnership 2007, paid a whopping $930 million for the buildings, borrowing almost all of it. What’s happened since then makes the deal a poster child for the troubles confronting Seattle’s commercial real-estate scene.