refactor(spec): Update Agent Card Well-Known Path to /.well-known/agent-card.json

Follow-up to a2aproject/A2A#841

Release-As: 0.3.0

Author: holtskinner

src/a2a/utils/constants.py

@@ -1,5 +1,5 @@
 """Constants for well-known URIs used throughout the A2A Python SDK."""
 
-AGENT_CARD_WELL_KNOWN_PATH = '/.well-known/agent.json'
+AGENT_CARD_WELL_KNOWN_PATH = '/.well-known/agent-card.json'
 EXTENDED_AGENT_CARD_PATH = '/agent/authenticatedExtendedCard'
 DEFAULT_RPC_URL = '/'