chore(api-core): remove dead __contains__ methods from MockRequest (#17796)

### Description
This PR addresses code review feedback by removing unused/dead mock
helper methods:

- Removed `__contains__` methods from `MockRequest` and
`MockValueErrorRequest` classes in `test_requests.py`, as the `in`
operator is only used on `dict` type checks inside `setup_request_id`.

Author: hebaalazzeh

packages/google-api-core/tests/unit/gapic/test_requests.py

@@ -27,9 +27,6 @@ def __init__(self, **kwargs):
         for k, v in kwargs.items():
             setattr(self, k, v)
 
-    def __contains__(self, key):
-        return hasattr(self, key)
-
 
 class MockProtoRequest:
     def __init__(self, **kwargs):
@@ -44,9 +41,6 @@ class MockValueErrorRequest:
     def HasField(self, key):
         raise ValueError("Mismatched field")
 
-    def __contains__(self, key):
-        return hasattr(self, key)
-
 
 # --- Parameterized Test ---