fix(identity): pin parity behavior + README example for A2A extensions

Test added asserting that build_a2a_agent_card omits the extensions field
when passed an empty list — locks down current behavior so it can't drift
from node-commerce's matching skip-empty rule (cross-language profiles
canonicalize to identical bytes when both omit).

README example updated to show ucp_a2a_extension() on the build call so
vendors copying the snippet ship spec-compliant A2A cards out of the box.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vvillait88

README.md

@@ -187,10 +187,14 @@ from agentscore_commerce.identity import (
     A2AAgentCardCapabilities,
     build_a2a_agent_card,
     build_ucp_profile,
+    ucp_a2a_extension,
 )
 
 # Google A2A v1.0 Signed Agent Card. Publish at /.well-known/agent-card.json.
-card = build_a2a_agent_card(name="My Service", url=base_url, capabilities=A2AAgentCardCapabilities(...), data=assess_result)
+# Per UCP §A2A binding the card MUST declare the canonical UCP extension URI;
+# pass `ucp_a2a_extension()` with empty capabilities until you bind formal UCP
+# capabilities (dev.ucp.shopping.checkout, etc.).
+card = build_a2a_agent_card(name="My Service", url=base_url, capabilities=A2AAgentCardCapabilities(...), extensions=[ucp_a2a_extension()], data=assess_result)
 
 # Google Universal Commerce Protocol. Publish at /.well-known/ucp.
 # Output shape: {"ucp": {"version", "services", "capabilities",

tests/test_a2a.py

@@ -144,3 +144,10 @@ def test_build_a2a_agent_card_emits_extensions_when_passed():
 def test_build_a2a_agent_card_omits_extensions_when_not_passed():
     card = build_a2a_agent_card(name="X")
     assert "extensions" not in card.to_dict()
+
+
+def test_build_a2a_agent_card_omits_extensions_when_passed_empty_list():
+    # Parity with node: both SDKs skip the extensions field when empty so
+    # cross-language profiles canonicalize to identical bytes when both omit.
+    card = build_a2a_agent_card(name="X", extensions=[])
+    assert "extensions" not in card.to_dict()