Improve YahooFinanceNewsTool to resolve tickers automatically

[axisrow]

Summary

• teach YahooFinanceNewsTool to accept company names and resolve them to tickers via yfinance.Search
• fetch and format news directly from Ticker.get_news() (no WebBaseLoader, better top_k handling)
• return compact sections with headline, summary, publisher, and link
• add unit test that mocks yfinance to cover both name→ticker resolution and direct ticker use

Testing

• uv sync --frozen --group lint --group test
• uv run --frozen ruff format langchain_community/tools/yahoo_finance_news.py
• uv run --frozen ruff check langchain_community
• uv run --frozen pytest tests/unit_tests/tools/test_yahoo_finance_news.py