[banco plugin] rework price feed

[louisinger]

the current price feed struct is fetching the first float in the response of any GET endpoint. it works for coingecko API but it is very fragile. we need a proper abstraction for the price feed, and an easy way to set it in the operator UI.

https://github.com/arkade-os/solver/blob/master/pkg/banco/price.go

[arkanaai]

Context

The fragile implementation is at internal/infrastructure/pricefeed/coingecko.go. CoinGecko.Fetch() deserializes the response into map[string]map[string]float64 and returns the first value from an unordered Go map — which happens to work for CoinGecko's {"bitcoin":{"usd":50000}} format but breaks for any other provider or response shape.

It's wired in at internal/solverd/runtime.go:setupBanco() as the default feed when none is injected. The banco.PriceFeed interface (pkg/banco/price.go) is already clean:

type PriceFeed interface {
    Fetch(ctx context.Context, feedURL string) (float64, error)
}

The interface is fine — the problem is that all extraction logic is hardcoded inside the single CoinGecko adapter with no way for an operator to point at Binance, Kraken, Coinbase, or any other endpoint.

Acceptance criteria

• A generic HTTPFeed adapter in internal/infrastructure/pricefeed/ that can extract a price from an arbitrary JSON response using a configurable selector (e.g. a dot-separated key path like bitcoin.usd or a JSONPath expression)
• Pair.PriceFeed (or a new Pair.PriceFeedPath) carries the extraction hint so operators can configure any REST endpoint without writing Go — or the URL encodes the path (e.g. https://api.coingecko.com/...#bitcoin.usd)
• CoinGecko either becomes a thin wrapper around HTTPFeed or is removed in favour of the generic adapter
• Existing unit tests for priceCache continue to pass; new tests cover at least CoinGecko-shaped and flat ({"price": 123.0}) responses
• The WithBancoPriceFeed override in runtime.go still works so tests/e2e can inject a mock

Suggested approach

The least-invasive path: add a PriceFeedPath string field to Pair (dot-separated JSON key path, e.g. bitcoin.usd). Implement HTTPFeed in pricefeed/ that does GET url, walks the decoded interface{} tree following the path, and returns the float. Replace NewCoinGecko() default in setupBanco with NewHTTPFeed(). The existing PriceFeed interface stays unchanged.

Alternatively, encode the path in the URL fragment (https://api.coingecko.com/...#bitcoin.usd) to avoid a schema change to Pair — simpler but less explicit.