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/samples/04-hosting/a2a/agent_definitions.py

@@ -10,7 +10,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-from a2a.types import AgentCapabilities, AgentCard, AgentSkill
+from a2a.types import AgentCapabilities, AgentCard, AgentInterface, AgentSkill
 from invoice_data import query_by_invoice_id, query_by_transaction_id, query_invoices
 
 if TYPE_CHECKING:
@@ -94,11 +94,11 @@ def get_invoice_agent_card(url: str) -> AgentCard:
     return AgentCard(
         name="InvoiceAgent",
         description="Handles requests relating to invoices.",
-        url=url,
         version="1.0.0",
         default_input_modes=["text"],
         default_output_modes=["text"],
         capabilities=_CAPABILITIES,
+        supported_interfaces=[AgentInterface(url=url, protocol_binding="JSONRPC")],
         skills=[
             AgentSkill(
                 id="id_invoice_agent",
@@ -116,11 +116,11 @@ def get_policy_agent_card(url: str) -> AgentCard:
     return AgentCard(
         name="PolicyAgent",
         description="Handles requests relating to policies and customer communications.",
-        url=url,
         version="1.0.0",
         default_input_modes=["text"],
         default_output_modes=["text"],
         capabilities=_CAPABILITIES,
+        supported_interfaces=[AgentInterface(url=url, protocol_binding="JSONRPC")],
         skills=[
             AgentSkill(
                 id="id_policy_agent",
@@ -138,11 +138,11 @@ def get_logistics_agent_card(url: str) -> AgentCard:
     return AgentCard(
         name="LogisticsAgent",
         description="Handles requests relating to logistics.",
-        url=url,
         version="1.0.0",
         default_input_modes=["text"],
         default_output_modes=["text"],
         capabilities=_CAPABILITIES,
+        supported_interfaces=[AgentInterface(url=url, protocol_binding="JSONRPC")],
         skills=[
             AgentSkill(
                 id="id_logistics_agent",