Fix shared provider config state across Client instances

[rakeshutekar]

Summary

This fixes a shared mutable default in Client.__init__ where multiple Client() instances could share the same provider_configs dict.

Problem

provider_configs previously defaulted to {} and configure() mutates that dict in place. As a result, configuring one default client could leak provider settings into another default client.

Changes

• Default provider_configs to None
• Create a fresh provider config dict per Client
• Copy provider config dictionaries on init and configure()
• Pass copied configs into lazy provider initialization
• Add regression tests for client config isolation

Test Plan

uv run --with pytest --with pydantic --with docstring-parser --with openai pytest tests/client/test_client.py

Result: 38 passed