test(siliconflow): add real release coverage

[lfnovo]

Summary

• add credential-gated real tests for SiliconFlow sync/async chat
• cover sync/async streaming and live model discovery
• document SILICONFLOW_API_KEY in the release credential reference

Validation

• env -u SILICONFLOW_API_KEY uv run pytest -m release tests/integration/test_chat_completion_real.py -k SiliconFlow -q --no-cov — 5 skipped cleanly, no API calls
• uv run ruff check tests/integration/test_chat_completion_real.py
• git diff --check

Live execution remains a maintainer-authorized Bucket C release check because it uses paid credentials.

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}