Add tests for generic capability detection and metadata extraction

[michalharakal]

Summary

Cover the new abstraction and detection layer with unit tests.

Scope

• Unit tests for capability/provider resolution
• Unit tests for metadata extraction from GGUF where applicable
• Unit tests for tokenizer/config chat template detection where applicable
• Unit tests for fallback selection behavior

Acceptance criteria

• Capability selection is deterministic and test-covered
• Missing/partial metadata cases are covered