fix(prices): Batch Alchemy token price requests to avoid 429

[sentry]

This PR addresses the AxiosError: Request failed with status code 429 issue observed on iOS, which was caused by exceeding Alchemy's 10,000 token_price requests/hour rate limit.

Root Cause:
The hooks/useBalances.ts file was making N parallel GET /by-symbol requests to Alchemy for N token symbols via Promise.allSettled(symbolsToFetch.map(s => fetchTokenPriceUsd(s))). Each fetchTokenPriceUsd call in lib/api.ts made a single HTTP request, leading to a fan-out that quickly exhausted the rate limit.

Solution:

1. lib/api.ts: Introduced a new exported function fetchTokenPricesUsdBatch(symbols: string[]). This function accepts an array of token symbols and makes a single GET /by-symbol request to Alchemy, utilizing the endpoint's ability to handle multiple symbols via repeated ?symbols= query parameters. It returns a Record<string, number> mapping symbols to their USD prices, gracefully handling unpriced or failed requests.
2. hooks/useBalances.ts: Modified the useBalances hook to replace the Promise.allSettled fan-out with a single call to fetchTokenPricesUsdBatch(symbolsToFetch). The import for fetchTokenPriceUsd was removed from this file as it's no longer directly used here.
3. lib/__tests__/alchemyTokenPrices.test.ts: Added comprehensive unit tests for fetchTokenPricesUsdBatch to ensure correct batching, parsing of responses, and error handling.

The existing fetchTokenPriceUsd function remains unchanged to support its other callers (e.g., useEstimateGas.ts, useNativePriceUsd.ts, useToken.tsx).

Fixes SOLID-ZZ

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