fix(event-handler): prevent OpenAPI schema bleed when reusing response dictionaries

Fixes #7711

When multiple routes shared the same response dictionary object,
the OpenAPI schema generator was mutating the shared dictionary
by directly modifying it. This caused schema bleeding where one
route's return type would incorrectly appear in another route's
OpenAPI schema.

The fix uses copy.deepcopy() to create independent copies of
response dictionaries before mutation, ensuring each route gets
its own correct OpenAPI schema based on its return type annotation.

Author: oyiz-michael

aws_lambda_powertools/event_handler/api_gateway.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import base64
+import copy
 import json
 import logging
 import re
@@ -666,7 +667,8 @@ def _get_openapi_path(  # noqa PLR0912
         # Add the response to the OpenAPI operation
         if self.responses:
             for status_code in list(self.responses):
-                response = self.responses[status_code]
+                # Create a deep copy to prevent mutation of the shared dictionary
+                response = copy.deepcopy(self.responses[status_code])
 
                 # Case 1: there is not 'content' key
                 if "content" not in response: