Handle null response output during parsing

[FU-max-boop]

Summary

Fixes #3325.

parse_response() currently assumes response.output is always iterable. Some streaming backends can send a final response.completed payload with output: null after emitting valid output-item events, which causes a bare TypeError in the response stream accumulator.

This change:

• treats a null final output as empty for standalone response parsing;
• preserves already accumulated streaming output when the final completion payload has output: null;\n- applies refusal, content-part, and output-item finalization events before recovering that output;
• covers both standalone parsing and the created/output-item/content-part/text-delta/completed streaming sequence.

The branch has been rebased onto the current main and the prior test-file conflict is resolved.

Tests

• pytest tests/lib/responses/test_responses.py -q — 11 passed
• ruff check src/openai/lib/_parsing/_responses.py src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_responses.py
• ruff format --check src/openai/lib/_parsing/_responses.py src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_responses.py
• scripts/run-pyright src/openai/lib/_parsing/_responses.py src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_responses.py
• git diff --check

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[FU-max-boop]

Rebased this branch onto the current main (a1eeab5) and resolved the test-file conflict while preserving both null-output regressions. The resolved review fix is now in a6aa9c2; focused tests (9), Ruff, Pyright, formatting, and git diff --check all pass locally.

[chatgpt-codex-connector]

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Reviewed commit: a6aa9c2a0a

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[marcuswood-oai]

Thank you for the contribution! The null-output parsing and stream recovery fix has landed in #3345, so I am closing this PR as superseded. We appreciate your help!