MarketDataClientErrorResult should be importable from package root

[MarketDataApp]

Problem

MarketDataClientErrorResult is a core user-facing type that every SDK user needs to handle errors, but it's not easily importable. Currently it lives in marketdata.sdk_error, which is not intuitive.

This has already caused documentation issues where examples incorrectly showed from marketdata.exceptions import MarketDataClientErrorResult.

Current State

from marketdata.client import MarketDataClient
from marketdata.sdk_error import MarketDataClientErrorResult  # Not intuitive

Proposed Solution

Add re-exports to __init__.py so users can write:

from marketdata import MarketDataClient, MarketDataClientErrorResult

These two types are always used together - you can't properly use the SDK without both. They should be importable from the same place.