fix: preserve HTTP response body after retries

[officialasishkumar]

Fixes: #1238

Summary

• add RetriesResult.Unwrap() so wrapped HTTP results remain discoverable through standard error traversal
• add regression coverage for unwrapping retries to *http.Result
• add a retry-path Send() test that checks the final response body is preserved

Testing

• go test ./protocol/http

[embano1]

@officialasishkumar needs rebase and signed-off commits.

[kunalworldwide]

Adding Unwrap() to RetriesResult fixes the error-traversal issue without breaking the existing API. The test shows that protocol.ResultAs(err, &result) now works after retries, which is the real-world use case from #1238.

Preserving the final response body in the retry path is also handled — the test confirms the body can still be read after Send returns. This avoids the common pitfall where wrapping a result and then retrying loses the original HTTP response for callers.

LGTM.