tests: FinnhubProvider coverage — stock, forex, _parse, error handling

[FaustoS88]

Adds pytest coverage for FinnhubProvider:

• _to_forex_symbol() — OANDA format conversion for all major pairs
• supports() — stocks, indices (^SPX), intl exchange-suffixed, forex, negative cases
• fetch() — stock happy path (values, limit, all intervals), forex routing to OANDA format
• routing guard — stock path never calls forex_candles, forex path never calls stock_candles
• fetch() error cases — unsupported interval (4h), missing API key, no-data status, client exception, None response
• _parse() — sync unit tests: candle values, limit enforcement, missing volume field, None input, empty timestamps

33 tests total. All passing on Python 3.11 and 3.12.