Fix: Surface non-2xx HTTP responses in client

[echobt]

Summary

The TCP-based HTTP client now parses the HTTP status line and returns a clear error for non-2xx responses instead of attempting to deserialize error bodies as successful JSON payloads.

Problem

Client::search and Client::embed_batch ignored the HTTP status line and always tried to deserialize the response body into the success shape. When the server returned 4xx/5xx responses, this produced confusing errors like "Failed to parse server response".

Solution

• Read and validate the HTTP status line.
• If the status code is not 2xx, return an error that includes the HTTP status code and, when available, the server-provided error field.
• Add regression tests covering both search and embed_batch non-2xx responses.

Testing

• cargo fmt --check
• CC=clang CXX=clang++ cargo test
• CC=clang CXX=clang++ cargo clippy --all-targets -- -D warnings

Related Issue

Fixes PlatformNetwork/bounty-challenge#73