fix(tck): resolve capability test failures for extensions and push notifications

[sherryfox]

• Default extensions to an empty list in AgentCapabilities to ensure it serializes as an array, fixing test_capabilities_structure.
• Save pushNotificationConfig in onMessageSend for non-streaming messages, fixing test_send_message_with_push_notification_config

Failures before the fixes:

========================================================= FAILURES ==========================================================
________________________________________________ test_capabilities_structure ________________________________________________
tests/optional/capabilities/test_agent_card_optional.py:87: in test_capabilities_structure
    assert isinstance(extensions, list), "extensions must be an array"
E   AssertionError: extensions must be an array
E   assert False
E    +  where False = isinstance(None, list)
______________________________________________ test_agent_extensions_structure ______________________________________________
tests/optional/capabilities/test_agent_card_optional.py:153: in test_agent_extensions_structure
    assert isinstance(extensions, list), "extensions must be an array"
E   AssertionError: extensions must be an array
E   assert False
E    +  where False = isinstance(None, list)
______________________________________ test_send_message_with_push_notification_config ______________________________________
tests/optional/capabilities/test_push_notification_config_methods.py:511: in test_send_message_with_push_notification_config
    assert found_config, (
E   AssertionError: Push notification config from SendMessageConfiguration was not found in task configs. Expected URL: http://localhost:8888/webhook. Retrieved 0 config(s): []. This indicates the pushNotificationConfig in SendMessageConfiguration was ignored.
E   assert False
----------------------------------------------------- Captured log call -----------------------------------------------------
ERROR    tests.optional.capabilities.test_push_notification_config_methods:test_push_notification_config_methods.py:507 ❌ Push notification config from SendMessageConfiguration was NOT found!
ERROR    tests.optional.capabilities.test_push_notification_config_methods:test_push_notification_config_methods.py:508 Expected URL: http://localhost:8888/webhook
ERROR    tests.optional.capabilities.test_push_notification_config_methods:test_push_notification_config_methods.py:509 Available configs: []
================================================== short test summary info ==================================================
FAILED tests/optional/capabilities/test_agent_card_optional.py::test_capabilities_structure - AssertionError: extensions must be an array
FAILED tests/optional/capabilities/test_agent_card_optional.py::test_agent_extensions_structure - AssertionError: extensions must be an array
FAILED tests/optional/capabilities/test_push_notification_config_methods.py::test_send_message_with_push_notification_config - AssertionError: Push notification config from SendMessageConfiguration was not found in task configs. Expected URL: http...
3 failed, 33 passed, 39 skipped, 1 xfailed in 14.86s

After the fixes all tests are passing:

================================================================================
📊 COMPREHENSIVE TEST RESULTS SUMMARY
================================================================================

🔴 Mandatory Tests:           ✅ PASSED (p:31/f:0/x:0/e:0/s:2)
🔄 Capability Tests:          ✅ PASSED (p:36/f:0/x:1/e:0/s:39)
🚀 Transport Equivalence:     ✅ PASSED (p:12/f:0/x:0/e:0/s:4)
🛡️  Quality Tests:             ✅ PASSED (p:14/f:0/x:0/e:0/s:0)
🎨 Feature Tests:             ✅ PASSED (p:15/f:0/x:0/e:0/s:1)

🎉 A2A COMPLIANCE: ✅ PASSED
Your implementation meets A2A specification requirements!
🔄 CAPABILITY HONESTY: ✅ EXCELLENT
All declared capabilities work correctly!
🛡️  PRODUCTION QUALITY: ✅ HIGH
Implementation is robust and production-ready!
🎨 FEATURE COMPLETENESS: ✅ COMPREHENSIVE
Implementation includes valuable optional features!

🏆 OUTSTANDING A2A IMPLEMENTATION!