[REFACTOR]: use NGNMarket ETF endpoint for ETF ticker resolution and validation

[aybruhm]

Summary

• Adds get_etf_detail() to NgnMarketAdapter which calls GET /etfs/{symbol} — the correct endpoint for ETF assets on NGNMarket (previously the index endpoint was always used as a fallback, even for ETFs)
• get_asset_metadata() now accepts an optional asset_class param; when "etf", it hits the ETF endpoint first and falls back to the index endpoint if the ETF endpoint returns nothing
• Threads asset_class through CsvImportInteractor._get_asset_metadata() so CSV imports route to the right endpoint based on the row's asset class column
• The GET /assets/validate endpoint now accepts an optional asset_class query param and forwards it to NGNMarket, so ticker validation also uses the correct endpoint
• Frontend validateTicker() passes the trade form's asset_class field when calling the validate endpoint

Test plan

• Import a CSV with NGNMarket ETF rows and confirm assets resolve correctly via /etfs/{symbol}
• Validate an NGNMarket ETF ticker from the trade form and confirm it returns supported: true
• Validate an NGNMarket non-ETF ticker (index) and confirm it still uses the index endpoint
• All unit and mocking tests pass (pytest tests/unit/adapters/outbound/market_data/test_ngnmarket_adapter.py tests/mocking/application/csv_import/)