fix: retain tool search and web fetch beta blocks in parsed streams

[nightcityblade]

Fixes #1422

Summary

• add the missing beta tool-search and web-fetch result block variants to ParsedBetaContentBlock
• cover both variants with focused construction tests so the parsed streaming union accepts them

Testing

• python3 -m venv .venv
• . .venv/bin/activate
• pip install -q inline_snapshot http_snapshot pytest pytest-xdist pytest_asyncio respx httpx pydantic
• pytest tests/test_models.py -k parsed_beta_content_block_accepts