refactor(server): 代码质量改进和 Go 1.21+ 现代化

[lwmacct]

Summary

• 使用 slices.Contains 替换手写循环查找逻辑（middleware, dashboard_events）
• 使用 maps.Copy 替换手写 map 复制循环（health.go）
• 修复 errcheck 警告：显式忽略 fmt.Sscanf 和 conn.Close 返回值
• 为预留的 registerDashboardRoutes 函数添加 //nolint:unused 指令
• 修复测试文件中的 errcheck 警告（loader_test.go, executor_test.go）

Changes

文件	修改
cmd/aster-server/main.go	显式忽略 fmt.Sscanf 错误
server/handlers/remote_agent.go	显式忽略 conn.Close() 错误
server/middleware.go	用 slices.Contains 替换循环
server/handlers/dashboard_events.go	用 slices.Contains 替换 2 处循环
server/observability/health.go	用 maps.Copy 替换循环
server/routes.go	保留预留函数，添加 //nolint:unused
pkg/config/loader_test.go	修复 os.Setenv/Unsetenv errcheck
pkg/executionplan/executor_test.go	修复 Execute errcheck

Test plan

• golangci-lint run --new 通过（0 issues）
• go build ./cmd/aster ./cmd/aster-server 通过
• pre-commit hooks 全部通过

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/sandbox/local.go	66.66%	2 Missing ⚠️

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