feat(anthropic): emit streaming latency span attrs

[sayonfortify]

Adds FortifyRoot TTFT/STTG span attributes to Anthropic streaming spans, matching the RDS-derived streaming latency contract already used by OpenAI.

What changed:

• Sync and async Anthropic stream wrappers now set:
  • fortifyroot.llm.streaming.time_to_first_token_ms
  • fortifyroot.llm.streaming.time_to_generate_ms
• TTFT is measured when the first content delta is received from the upstream stream, before FortifyRoot's one-item safety buffer can add yield-time delay.
• STTG is measured from first token to stream completion.
• Empty/no-token streams omit both attrs, preserving nullable backend semantics instead of emitting fake zeroes.

Tests:

• PYTHONPATH=packages/opentelemetry-instrumentation-anthropic:packages/opentelemetry-instrumentation-fortifyroot conda run -n py3.12 python -m pytest packages/opentelemetry-instrumentation-anthropic/tests/test_safety_unit.py -q → 19 passed
• python3 -m py_compile streaming.py test_safety_unit.py
• conda py3.12 py_compile same files
• git diff --check clean

No vendored ocelle-py changes in this PR. After merge, cut the next fr-v0.52.6.N tag and vendor via ocelle-py/scripts/vendor.sh per internal SOP.