fix(openai): support Langfuse arguments in stream helpers

[gigio1023]

What does this PR do?

Allows the native Chat Completions and Responses stream() helpers to accept Langfuse-specific arguments. The inner create(stream=True) call remains the single owner of the generation observation.

• Covers sync and async Chat helpers from OpenAI 1.40, using the legacy beta registration before 1.92, and Responses helpers from 1.66.
• Preserves native stream managers, structured-output and final-result helpers, user-provided extra_body, and sync manager reuse.
• Finalizes each observation once on exhaustion, explicit close, or context-manager exit after partial or no consumption.
• Treats OpenAI Omit defaults as absent so inputs, metadata, and model parameters remain clean. Actual Responses retrieval streams remain uninstrumented.

Fixes langfuse/langfuse#15018

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

uv run --frozen pytest -q tests/unit/test_openai.py tests/unit/test_openai_stream_compatibility.py
for version in 1.40.0 1.66.0 1.92.0; do uv run --frozen --with "openai==$version" pytest -q tests/unit/test_openai_stream_compatibility.py; done
LANGFUSE_PUBLIC_KEY=test-public-key LANGFUSE_SECRET_KEY=test-secret-key LANGFUSE_BASE_URL=http://localhost:3000 bash scripts/codex/quick-check.sh

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed.
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

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