fix(http): preserve unexpected response context

[huacnlee]

Summary

• preserve the HTTP status, x-trace-id, complete response headers, and raw body when a response cannot be parsed as the standard OpenAPI envelope
• expose the preserved context through a new UnexpectedHttpResponse error variant while retaining the existing BadStatus variant for compatibility
• keep rich 429 responses retryable and return the final original response when retries are exhausted
• document the behavior change in the changelog

This makes intermediary and load-balancer failures diagnosable instead of reducing responses such as HTTP 463 to status error: 463 <unknown status code>.

Test Plan

• cargo +nightly fmt --all
• cargo test -p longbridge-httpcli
• cargo clippy --all --all-features
• git diff --check

The full workspace Clippy run completes successfully; its warnings are pre-existing in current main and unrelated to this change.