fix(yfinance): normalize class-share symbols before Yahoo requests

[gyx09212214-prog]

Summary

• Add yfinance-only class-share symbol normalization for common US class-share symbols such as BRK.A and BF.B, converting them to Yahoo's dash-form symbols at the request boundary.
• Apply the helper to yf_download(...) and yfinance Ticker(...)-based equity/fundamental request paths while preserving the requested dot-form symbol in OpenBB symbol output fields.
• Add mocked coverage for helper boundaries, yf.download ticker arguments, multi-symbol historical output, and a quote fetcher Ticker("BRK-A") path.

Why

Users can use OpenBB's dot convention from #6702 while Yahoo still receives the dash-form symbols it expects. This keeps the mapping vendor-specific and avoids changing global symbol semantics.

Scope

• Only openbb-yfinance is changed.
• No standard model changes.
• No global symbol policy changes.
• No non-yfinance provider changes.
• The conversion is deliberately conservative for this first step, covering common class suffixes A, B, and C so one-letter Yahoo exchange suffixes such as VOD.L remain unchanged.

Validation

From openbb_platform/providers/yfinance:

• python -m pytest tests/test_yfinance_helpers.py -q - 19 passed
• python -m ruff check openbb_yfinance tests/test_yfinance_helpers.py - passed

Refs #6702