chore(vendor): sync OpenAI streaming latency span attrs

[sayonfortify]

Carries FortifyRoot/openllmetry-py#34 into the vendored OpenAI instrumentation.

What changed:

• Syncs OpenAI streaming TTFT/STTG span attributes into _vendor.
• Preserves existing Mimir histogram recording.
• First-token detection is independent of histogram availability, so tracing/RDS span attrs work even when metrics histograms are disabled.
• Empty streams still set neither attr, preserving nullable RDS semantics.

Scope:

• Single vendored file only.
• No release/tag/PyPI work.
• No unrelated vendor churn or VENDOR_MANIFEST update in this pre-release sync.

Validation:

• /Users/sayondutta/opt/anaconda3/envs/py3.12/bin/python -m py_compile src/fortifyroot/_vendor/opentelemetry/instrumentation/openai/shared/chat_wrappers.py
• git diff --check clean.

Source PR merged first: FortifyRoot/openllmetry-py#34.