fix: resolve CI linting and code quality issues

- Remove unused imports across all generator files
- Remove unused variables (spec, url_params, query_params, test_name)
- Fix f-strings without placeholders in auth.py and go.py
- Fix duplicate BASIC auth handling with wrong indentation
- Add missing pytest fixtures (sample_openapi_spec, temp_spec_file, temp_json_spec_file)
- Add missing TemplateRenderError import to generator files

api_testgen/mocks/generator.py

@@ -2,7 +2,7 @@
 
 import json
 from pathlib import Path
-from typing import Any, Dict, List, Optional
+from typing import List, Optional
 
 from ..core import SpecParser