fix: guard zero context length display + add 19 tests for model info
- ModelInfoCard: hide card when effective_context_length <= 0 instead
of showing 'Context Window: 0 auto-detected'
- Add tests for _normalize_config_for_web model_context_length extraction
- Add tests for _denormalize_config_from_web round-trip (write back,
remove on zero, upgrade bare string to dict, coerce string input)
- Add tests for CONFIG_SCHEMA ordering (model_context_length after model)
- Add tests for GET /api/model/info endpoint (dict config, bare string,
empty model, capabilities, graceful error handling)
62fb6b2cd8