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/core/spec_parser.py

@@ -6,7 +6,6 @@ from pathlib import Path
 from typing import Any, Dict, List, Optional, Union
 
 from openapi_spec_validator import validate
-from openapi_spec_validator.versions import consts as validator_consts
 
 from .exceptions import InvalidOpenAPISpecError, UnsupportedVersionError