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/auth.py

@@ -1,6 +1,6 @@
 """Authentication configuration for API TestGen."""
 
-from typing import Any, Dict, List, Optional, Union
+from typing import Any, Dict, List, Optional
 from enum import Enum
 
 from .exceptions import AuthConfigError, MissingSecuritySchemeError
@@ -293,21 +293,21 @@ func getAPIKeyHeaders() map[string]string {{
 }}
 '''
         elif method["type"] == AuthType.BEARER:
-            return f'''
-func getBearerHeaders() map[string]string {{
-    return map[string]string{{
+            return '''
+func getBearerHeaders() map[string]string {
+    return map[string]string{
         "Authorization": fmt.Sprintf("%s %s", os.Getenv("TOKEN_PREFIX"), os.Getenv("TOKEN")),
-    }}
-}}
+    }
+}
 '''
         elif method["type"] == AuthType.BASIC:
-            return f'''
-func getBasicHeaders(username, password string) map[string]string {{
+            return '''
+func getBasicHeaders(username, password string) map[string]string {
     auth := username + ":" + password
     encoded := base64.StdEncoding.EncodeToString([]byte(auth))
-    return map[string]string{{
+    return map[string]string{
         "Authorization": "Basic " + encoded,
-    }}
-}}
+    }
+}
 '''
         return ""