chore(lint): enable modernize linter and apply suggested refactorings

[dongjiang1989]

Summary

Enable the modernize linter in .golangci.yml and apply the refactorings it suggests across the codebase.

Changes

• .golangci.yml: enable modernize in the linter list; also fix missing trailing newline.
• a2acompat/a2av0/migration.go: replace manual map copy loop with maps.Collect.
• a2acompat/a2av0/rest_compat_test.go: replace strings.HasPrefix + strings.TrimPrefix pair with strings.CutPrefix.
• a2asrv/eventqueue/manager_in_memory_impl_test.go and a2asrv/push/store_test.go: replace for i := 0; i < n; i++ with for i := range n.
• a2asrv/taskstore/inmemory.go and a2asrv/taskstore/store_test.go: replace []byte(fmt.Sprintf(...)) with fmt.Appendf(nil, ...).

Verification

• go vet ./... — clean
• golangci-lint run ./... — 0 issues
• go test -count=1 ./... — all 29 packages pass