fix: replace generic Error with typed AlexSDKError and fix fetchData bugs

[vwinee21]

Summary

Replace all throw new Error(...) across the SDK with a typed AlexSDKError class, and fix two bugs in fetchData.ts.

Changes

src/errors.ts (new)

• AlexSDKError class with type, title, status, detail fields — inspired by RFC 9457 Problem Details
• AlexErrorType const enum: FetchFailed, RouteNotFound, TooManyPools, TokenMappingNotFound

src/utils/fetchData.ts — bug fixes

• getPrices() was throwing 'Failed to fetch token mappings' — correct message is 'Failed to fetch token prices'
• getPrices() used any for response shape — replaced with typed RawPriceEntry / RawPricesResponse
• HTTP status code from upstream now included in error detail

All helpers + alexSDK.ts, postConditions.ts, utils.ts

• All throw new Error(...) replaced with typed AlexSDKError

src/index.ts

• Exports AlexSDKError and AlexErrorType for consumers

Why

Generic throw new Error() forces consumers to parse message strings to distinguish error types. With AlexSDKError:

try {
  await sdk.getRate(Currency.STX, Currency.ALEX, 1_000_000n)
} catch (e) {
  if (e instanceof AlexSDKError && e.type === AlexErrorType.RouteNotFound) {
    // handle gracefully
  }
}