fix(ai-adapters): surface provider code 1305 in OpenAI stream errors

[cooleryu]

Summary

Preserve provider error metadata from OpenAI-compatible SSE error payloads.

When a provider sends an SSE data payload like:

{"error":{"code":"1305","message":"provider temporarily overloaded"},"request_id":"req_1305"}

BitFun already recognizes it as an API error before deserializing it as a normal chat chunk. This follow-up keeps the provider code and request_id in the surfaced error message instead of reducing the error to message-only text.

Motivation

Issue #380 shows a provider overload response where the useful debugging fields were:

• code
• message
• request_id

Keeping those fields visible makes model overload, quota, gateway, and provider-side failures easier to diagnose from BitFun's runtime errors and raw SSE logs.

Changes

• Include error.code in OpenAI-compatible SSE API error messages when present.
• Include top-level request_id / requestId when present.
• Keep existing behavior unchanged for message-only and string-shaped error payloads.
• Add an adapter unit test for the code + message + request_id shape.
• Add a bitfun-core stream fixture test so existing CI covers the end-to-end StreamProcessor error path.

Test Plan

git diff --check

Result: passed.

Attempted focused Rust tests:

cargo test -p bitfun-ai-adapters stream::stream_handler::openai::tests::extracts_api_error_code_message_and_request_id_from_object_shape
cargo test -p bitfun-core --test stream_processor_openai openai_fixture_preserves_provider_error_code_and_request_id

Result: not run locally because this machine does not currently have cargo / rustc installed. The new core integration test is under src/crates/core/tests, which is covered by the existing Linux CI job:

cargo test --locked -p bitfun-core

Risk

Low. This only changes formatting for OpenAI-compatible stream error payloads that already contain an error object. Normal chat completion chunks, tool-call chunks, and message-only error payloads keep their current behavior.

Related Issue

Refs #380.

[wsp1911]

需要将provider_error_with_code.sse内容改为

data: {"error":{"code":"1305","message":"provider temporarily overloaded"},"request_id":"req_1305"}