Add request retry preparation

[ioquatix]

Summary

• Add Protocol::HTTP::Request#rewind! to prepare a request body to be sent again, independent of request method semantics.
• Add Protocol::HTTP::Request#retry! to prepare requests for safe ambiguous retries by checking method semantics and delegating to rewind!.
• Preserve the existing Request#idempotent? behavior for compatibility with existing clients.
• Cover rewind/retry preparation for no body, rewindable bodies, non-rewindable bodies, and non-idempotent methods.

Intended Usage

• Use request.rewind! when the transport knows the previous attempt was not processed, e.g. Protocol::HTTP::RefusedError.
• Use request.retry! when retrying an ambiguous failure, where method semantics matter.

Testing

• bundle exec sus test/protocol/http/request.rb
• bundle exec sus