test(anthropic): add null citation streaming cassette

[gold-silver-copper]

Summary

• Add an Anthropic streaming document-citation cassette that exercises the null citations content_block_start shape from cannot deserialize anthropic resp stream if citations in stream is null #1971/fix(anthropic): deserialize explicit null citations as empty vec #1972
• Verify the end-to-end streaming flow still returns citation-backed text instead of failing during SSE deserialization

Follows up #1972.

Tests

• cargo test -p rig --all-features --test anthropic anthropic::cassette::plaintext_document -- --nocapture --test-threads=1
• cargo test -p rig --all-features --test anthropic cassette_safety -- --nocapture