fix(streaming): wrap midstream transport errors in APITimeoutError / APIConnectionError (#3811)

[katariyaVivek]

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Problem

When consuming a streaming response (e.g. client.chat.completions.create(..., stream=True)), transport errors that occur mid-stream (such as a read timeout or dropped socket connection) escaped as raw httpx.ReadTimeout, httpx2.ReadTimeout, or httpx2.RemoteProtocolError.

Because these exceptions are not subclasses of openai.APIError (they do not inherit from APIConnectionError or APITimeoutError), standard exception-handling blocks around API calls:

try:
    for chunk in client.chat.completions.create(..., stream=True):
        ...
except openai.APIError as err:
    ...

fail to catch the most common streaming failures, breaking client resilience and leaving mid-stream errors inconsistent with the initial request-send path.

Root Cause

Stream.__stream__() and AsyncStream.__stream__() in src/openai/_streaming.py iterated over SSE events without exception-wrapping blocks. Unlike _base_client.py (which maps timeout_exceptions() to APITimeoutError and transport exceptions to APIConnectionError), the streaming iterator let raw httpx2 / legacy httpx exceptions bubble out directly to the consumer.

Solution

1. In src/openai/_streaming.py, wrapped the iteration loop in Stream.__stream__() and AsyncStream.__stream__():
   • except timeout_exceptions() as err: -> raise APITimeoutError(request=request) from err
   • except _transport_exceptions() as err: -> raise APIConnectionError(request=request) from err
   • _transport_exceptions() covers httpx2.TransportError and httpx.TransportError (when legacy httpx is loaded), matching the timeout_exceptions() pattern.
   • response.close() / await response.aclose() is safely retained in the finally: block.
2. In src/openai/lib/streaming/_assistants.py, handled APITimeoutError and APIConnectionError in AssistantEventHandler and AsyncAssistantEventHandler, unwrapping exc.__cause__ so that existing on_timeout() and on_exception() callbacks receive the underlying transport exception family, preserving the contract tested by test_assistant_stream_timeout_callbacks_preserve_httpx2_family.

Testing

Added 4 new test cases to tests/test_httpx2.py:

• test_chat_stream_midstream_timeout_wrapped (sync ReadTimeout -> APITimeoutError)
• test_chat_stream_midstream_connection_error_wrapped (async RemoteProtocolError -> APIConnectionError)
• test_chat_stream_midstream_connection_error_wrapped_sync (sync RemoteProtocolError -> APIConnectionError)
• test_chat_stream_midstream_timeout_wrapped_async (async ReadTimeout -> APITimeoutError)

Verified:

• Fails pre-fix (httpx2.ReadTimeout and httpx2.RemoteProtocolError escape raw).
• Passes post-fix with __cause__ correctly chained.
• tests/test_httpx2.py and tests/test_streaming.py pass cleanly (41 passed, 1 skipped).

Verification

• Targeted tests: 4 passed in tests/test_httpx2.py
• Integration tests: 41 passed across tests/test_httpx2.py and tests/test_streaming.py
• Type checking: Verified against _constants.py, _httpx2.py, and _exceptions.py
• Lint: ruff check passed with 0 errors
• Formatter: ruff format --check passed (3 files checked)

Impact

Consumers iterating over streams can now reliably catch openai.APIError, openai.APIConnectionError, and openai.APITimeoutError for all midstream transport failures, consistent with the rest of the SDK.

Fixes #3811