refactor: apply the Go 1.27 go fix modernizers

[tas50]

Summary

Runs go fix ./... (Go 1.27) over both modules and keeps everything it suggests. All are behavior-preserving rewrites to current idiom:

Rewrite	Where
errors.As(err, &v) → errors.AsType[T](err)	transport.go (isRetriable), acls_grant_test.go
strings.IndexByte + slice → strings.Cut	transport.go (stripping the query from the signing path)
range over strings.Split → strings.SplitSeq	chefignore.go (no intermediate slice)
header copy loop → maps.Copy	transport.go, integration/suite/raw.go
atomic.AddInt64(&n) → atomic.Int64	cookbooks_test.go, transport_test.go
if/else clamp → min	search_test.go

No public API change.

Test plan

• go test ./... -race -count=2 clean
• cd integration && go test ./... passes (HTTP and TLS targets)
• golangci-lint run ./... clean in both modules
• Read every production-code hunk: the signing path, header copy and retry classification are unchanged