test: Add comprehensive ModelRegistry class tests (34 tests)

Boost test coverage for ModelRegistry OOP interface:
- Initialization with default and custom registries
- Cache building and population verification
- get_model_by_id() with O(1) cache lookup
- get_models_by_tier() with O(1) cache lookup
- get_model() with provider validation
- list_providers() and list_tiers()
- get_all_models() registry access
- get_pricing_for_model() pricing lookup
- Performance verification for cache hits

Coverage focus: Methods not fully tested by helper function tests.
Result: 34 tests passing, comprehensive coverage of ModelRegistry class.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: silversurfer562