Refactor client module into package

[tconley1428]

Summary

• Replace monolithic temporalio/client.py with a temporalio/client package
• Split client functionality by area: client facade, workflows, activities, schedules, interceptors, implementation, helpers, worker versioning, cloud, plugin, exceptions, and callback alias
• Preserve temporalio.client import compatibility, including the private _history_from_json helper used by tests

Validation

• poe lint
• uv run pytest tests/test_client.py tests/test_client_type_errors.py tests/test_activity.py tests/test_serialization_context.py tests/test_plugins.py tests/test_cloud.py -q