Add re-exports to package root for core types

[MarketDataApp]

Summary

• Export MarketDataClient, MarketDataClientErrorResult, OutputFormat, DateFormat, and Mode from the package root
• Update all documentation examples to use the cleaner import pattern

Users can now write:

from marketdata import MarketDataClient, MarketDataClientErrorResult, OutputFormat, DateFormat, Mode

Instead of:

from marketdata.client import MarketDataClient
from marketdata.sdk_error import MarketDataClientErrorResult
from marketdata.input_types.base import OutputFormat, DateFormat, Mode

Test plan

• All 254 tests pass with 100% coverage
• Verified new imports work correctly
• Verified old import paths still work (backwards compatible)

Fixes #16

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (aa19383) to head (3c41fe5).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           51        52    +1     
  Lines         2205      2209    +4     
=========================================
+ Hits          2205      2209    +4     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.