fix: handle content_block_delta streaming deserialization fallback

[nightcityblade]

Fixes #941

Summary

• add a targeted fallback when a streamed content_block_delta event remains a raw dict after the generic union deserialization path
• apply the same fallback in the beta streaming accumulator
• add regression tests covering dict-shaped content_block_delta events in both standard and beta streaming paths

Testing

• uv run ruff check src/anthropic/lib/streaming/_messages.py src/anthropic/lib/streaming/_beta_messages.py tests/lib/streaming/test_messages.py tests/lib/streaming/test_partial_json.py
• uv run pytest tests/lib/streaming/test_messages.py tests/lib/streaming/test_partial_json.py -q -n 0