Surface streaming HTTP errors at the function call instead of as first stream item

[max-zheng]

Surfaces non-2xx HTTP responses as Err(OpenAIError::ApiError(_)) from the streaming methods (post_stream, post_stream_mapped_raw_events, get_stream) instead of delivering them as the first item of the returned stream. Centralizes the status check in execute_response so all callers (unary and streaming) share one code path for "non-2xx → error."

Closes #553.

Changes

• execute_response now bails with read_error_response(response).await on non-success status, so every downstream caller sees only 2xx responses.
• New read_error_response helper consumes a non-2xx response and produces the matching OpenAIError::ApiError(ApiErrorResponse { .. }). It's the single source of truth for HTTP error construction — previously this logic was duplicated inline in read_response and (incorrectly, via an InvalidArgument fallback) in the wasm stream_mapped_raw_events.
• read_response, execute_stream, execute_stream_mapped_raw_events, and both stream_mapped_raw_events impls drop their now-redundant local status checks.
• Removes the OpenAIError::InvalidArgument(...) fallback from the wasm streaming path (per maintainer feedback on Title: create_stream* methods return Ok(stream) on HTTP error; error surfaces as first stream item instead of as Err from the call #553 — InvalidArgument is reserved for client-side validation).

[64bit]

This is so nice, thank you!