Remove unreachable SSE error branch

[LOGO127]

Summary

• Remove the unreachable sse.event == "error" checks nested under the thread. event branch in both Stream and AsyncStream.
• Preserve the existing reachable error handling for non-thread. SSE payloads.

Closes #2796

Tests

• pytest -o addopts='' tests/test_streaming.py -q (20 passed)
• ruff check src/openai/_streaming.py
• ruff format --check src/openai/_streaming.py
• git diff --check

[LOGO127]

Maintainers, this PR intentionally keeps the diff to 28 deleted lines and leaves the reachable SSE error handling unchanged. The focused streaming tests and Ruff checks pass. Happy to adjust if the repository prefers an additional regression test.

[LOGO127]

@openai/sdks-team, could a code-owner reviewer take a look when convenient? This PR removes only the duplicated unreachable error branch under thread.*; the focused streaming tests and Ruff checks pass. Happy to adapt if the SDK team prefers a regression test or a different cleanup scope.

[sylvesterkaczmarek]

Correct cleanup. sse.event == "error" is unreachable under the thread. prefix branch, while the non-thread path still owns SSE error propagation. Removing both sync and async copies reduces misleading dead logic without changing event semantics.

[LOGO127]

Thanks for confirming the event semantics. No code changes are needed from this review. @openai/sdks-team, could a code-owner maintainer record the formal approval when convenient?