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/generators/pytest.py

@@ -7,7 +7,7 @@ from typing import Any, Dict, List, Optional
 from jinja2 import Environment, FileSystemLoader, TemplateSyntaxError, UndefinedError
 
 from ..core import SpecParser, AuthConfig
-from ..core.exceptions import GeneratorError, TemplateRenderError
+from ..core.exceptions import TemplateRenderError
 
 
 class PytestGenerator: