Fix stacked content encoding decoding

[ryanfowler]

Summary

• Parse all Content-Encoding header values, including comma-delimited stacks.
• Decode supported stacked encodings in reverse order.
• Leave unsupported encoding stacks untouched to avoid partial decoding.
• Add tests for stacked, repeated-header, and unsupported encoding cases.

Testing

• go test -v ./internal/client
• go test -v ./integration -run 'TestMain/(gzip compression|zstd compression)'