Restore config singleton and fix package declarations

[skamanbo]

Summary

• restore the thread-safe singleton behavior for Config and AsyncConfig
• retain warnings when later construction requests conflict with the initialized configuration
• restore positional argument, region alias, and URL normalization checks
• remove nonexistent top-level Poetry package declarations while retaining all validation SDK modules under aidefense
• add an explicit regression test for singleton identity

Root cause

PR #113 removed the singleton implementation from BaseConfig while adding the validation SDK. The existing configuration tests and downstream callers still rely on the singleton contract. The same PR also declared nonexistent top-level packages, which prevented poetry install from installing the project.

Impact

This restores the SDK's previous configuration behavior without removing or modifying the validation SDK. A built wheel contains all 24 modules under aidefense.validation and aidefense.pydantic.validation.

Validation

• poetry check
• poetry run pytest aidefense/tests/test_config.py - 15 passed
• poetry run pytest - 600 passed, 8 skipped
• built the wheel and verified validation modules are included
• git diff --check