A bitter 15-year legal battle over water costs came to an end Monday, with leaders of the San Diego County Water Authority and the Metropolitan Water District of Southern California signing an agreement establishing the price that will be paid for delivering supplies. Managers and board members of the two agencies said that the dispute persisted for years because of inflexible positions, but that negotiations over the last year made possible a comprehensive agreement. They said ending the legal fight will enable greater collaboration among the agencies to improve their finances and move water where it’s needed.