President Barack Obama has rejected a Canadian company’s request to build the Keystone XL Pipeline, which would have carried 800,000 barrels of oil a day from Alberta’s tar sands to refineries on the U.S. Gulf Coast. The decision is a major victory for climate and conservation groups and burnishes Obama’s legacy in the battle to slow global warming. Obama’s announcement, made after a seven-year review by the U.S. State Department and other agencies, comes just weeks ahead of key United Nations climate talks in Paris. In remarks at the White House, Obama said that the economic benefits of building the pipeline were outweighed by the high environmental costs of helping move to market tar sands crude, whose production is among the most carbon-polluting on the planet. “The pipeline would not make a meaningful long-term contribution to our economy,’’ Mr. Obama said in remarks at the White House.