2c719f0701
Anthropic migrated their OAuth infrastructure from console.anthropic.com to platform.claude.com (Claude Code v2.1.81+). Update _refresh_oauth_token() to try the new endpoint first, falling back to the old one for tokens issued before the migration. Also switches Content-Type from application/x-www-form-urlencoded to application/json to match current Claude Code behavior. Salvaged from PR #2741 by kshitijk4poor.