Python: [BREAKING] Migrate agent-framework-a2a to a2a-sdk v1.0 (#5752)

* Python: Migrate agent-framework-a2a to a2a-sdk v1.0

Upgrade the a2a-sdk dependency from v0.3.x to v1.0.0 and migrate all
source, tests, samples, and documentation to the v1.0 API.

Key changes:
- Dependency: a2a-sdk>=1.0.0,<2 (was >=0.3.5,<0.3.24)
- Types are now protobuf-based: Part replaces TextPart/FilePart/DataPart
- Enums use SCREAMING_SNAKE_CASE (e.g. TaskState.TASK_STATE_COMPLETED)
- Roles: Role.ROLE_AGENT, Role.ROLE_USER
- Client: SendMessageRequest wrapper, subscribe() replaces resubscribe()
- Server: A2AStarletteApplication replaced by Starlette + route factories
- DefaultRequestHandler now requires agent_card parameter
- TaskUpdater: final parameter removed, add_artifact gains last_chunk
- AgentCard.url removed; use supported_interfaces with AgentInterface
- Stream yields StreamResponse with WhichOneof('payload')

Closes #5661

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review: validate fallback URL, remove unused task_id vars

- Raise ValueError with clear message when transport negotiation fails
  and no fallback URL is available (neither url arg nor supported_interfaces)
- Remove unused task_id local in status_update branch
- Inline artifact_event.task_id directly in artifact_update branch

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/a2a/AGENTS.md

@@ -23,23 +23,28 @@ response = await a2a_agent.run("Hello!")
 
 ```python
 from agent_framework.a2a import A2AExecutor
-from a2a.server.apps import A2AStarletteApplication
 from a2a.server.request_handlers import DefaultRequestHandler
+from a2a.server.routes import create_agent_card_routes, create_jsonrpc_routes
 from a2a.server.tasks import InMemoryTaskStore
+from starlette.applications import Starlette
 
 # Create an A2A executor for your agent
 executor = A2AExecutor(agent=my_agent)
 
-# Set up the request handler and server application
+# Set up the request handler (agent_card is required)
 request_handler = DefaultRequestHandler(
     agent_executor=executor,
     task_store=InMemoryTaskStore(),
+    agent_card=my_agent_card,
 )
 
-app = A2AStarletteApplication(
-    agent_card=my_agent_card,
-    http_handler=request_handler,
-).build()
+# Build a Starlette app with A2A routes
+app = Starlette(
+    routes=[
+        *create_agent_card_routes(my_agent_card),
+        *create_jsonrpc_routes(request_handler),
+    ]
+)
 ```
 
 ## Import Path