fix(bdd): correct a2a SDK client class name in tdd_a2a_sdk_dependency test

The scenario "a2a SDK provides the A2AClient class" was checking for
A2AClient in a2a.client, but the installed a2a-sdk package exposes the
class as Client (not A2AClient). Updated the test steps to access and
assert on Client, which is the actual abstract base class provided by
the SDK.

ISSUES CLOSED: #4273

features/tdd_a2a_sdk_dependency.feature

@@ -19,5 +19,5 @@ Feature: A2A Python SDK is a declared project dependency
 
   @tdd_issue @tdd_issue_4273
   Scenario: a2a SDK provides the A2AClient class
-    When I import "a2a.client" and access "A2AClient"
-    Then the "A2AClient" class should be available
+    When I import "a2a.client" and access "Client"
+    Then the "Client" class should be available