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/pyproject.toml

@@ -24,7 +24,7 @@ classifiers = [
 ]
 dependencies = [
     "agent-framework-core>=1.3.0,<2",
-    "a2a-sdk>=0.3.5,<0.3.24",
+    "a2a-sdk>=1.0.0,<2",
 ]
 
 [tool.uv]