fix: pass agent_card to DefaultRequestHandler (a2a-sdk 1.x requirement)

a2a-sdk 1.x added agent_card as a required argument to
DefaultRequestHandler.__init__. main.py constructed it with only
agent_executor + task_store, so every workspace startup that reached
the handler init step crashed with:

  TypeError: DefaultRequestHandlerV2.__init__() missing 1 required
  positional argument: 'agent_card'

This is the 6th a2a-sdk migration find from the v0 → v1 transition
(see reference_a2a_sdk_v0_to_v1_migration memory). Pattern is the
same: SDK exposes a new required arg, our call site needs to pass
the existing object we already construct upstream.

Why the import-only smoke gates didn't catch this: it's a call-time
constructor error inside `async def main()`, not a module load
error. The runtime-pin-compat smoke imports main_sync but doesn't
invoke main() against a real config. Worth filing a follow-up to
extend the smoke to a "construct + dispose" cycle.

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

workspace/main.py

@@ -233,6 +233,13 @@ async def main():  # pragma: no cover
     handler = DefaultRequestHandler(
         agent_executor=executor,
         task_store=InMemoryTaskStore(),
+        # a2a-sdk 1.x added agent_card as a required positional/keyword
+        # argument — it's used internally for capability dispatch (e.g.
+        # routing tasks/get historyLength based on the card's protocol
+        # version). Pass the same agent_card we registered with the
+        # platform so the handler's capability surface matches what the
+        # AgentCard advertises.
+        agent_card=agent_card,
     )
 
     # v1: replace A2AStarletteApplication with Starlette route factory