Add QueryBuilder.to_dataframe() for fluent DataFrame queries #147

Summary

Add a to_dataframe() method to QueryBuilder to connect the DataFrame scenario to the fluent syntax / QueryBuilder changes.

This enables a workflow like:

df = client.query.builder("account").select("name", "telephone1").filter("statecode eq 0").top(100).to_dataframe()

Context

• Identified as a followup in PR Add client.dataframe namespace for pandas DataFrame CRUD operations #98 by tpellissier-msft
• Depends on the QueryBuilder/fluent syntax refactor landing first
• The client.dataframe.get() method already provides DataFrame-native query support, but to_dataframe() would connect it to the fluent QueryBuilder API

Acceptance Criteria

• QueryBuilder has a to_dataframe() terminal method that executes the query and returns a pd.DataFrame
• All pages are consolidated into a single DataFrame (same behavior as client.dataframe.get())
• Unit tests covering the new method
• Documentation updated (README, docstrings)