[BUG] A2A Doesn't Work on 1.2.0

### Checks

- [x] I have updated to the lastest minor and patch version of Strands
- [x] I have checked the documentation and this is not expected behavior
- [x] I have searched [./issues](./issues?q=) and there are no duplicates of my issue

### Strands Version

1.2.0

### Python Version

3.12.7

### Operating System

mac0S 15.5

### Installation Method

pip

### Steps to Reproduce

Run this example server

https://strandsagents.com/latest/documentation/docs/user-guide/concepts/multi-agent/agent-to-agent/#basic-server-setup

```
import logging
from strands_tools.calculator import calculator
from strands import Agent
from strands.multiagent.a2a import A2AServer

logging.basicConfig(level=logging.INFO)

# Create a Strands agent
strands_agent = Agent(
    name="Calculator Agent",
    description="A calculator agent that can perform basic arithmetic operations.",
    tools=[calculator],
    callback_handler=None
)

# Create A2A server (streaming enabled by default)
a2a_server = A2AServer(agent=strands_agent)

# Start the server
a2a_server.serve()
```

### Expected Behavior

```
❯ python 4_a2a_server.py                  
INFO:     Started server process [92853]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:9000 (Press CTRL+C to quit)
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
INFO:     Finished server process [92853]
```

This works on Strands 1.1.0

### Actual Behavior

```
❯ python 4_a2a_server.py              
Strands A2A server encountered exception.
Traceback (most recent call last):
  File "/Users/mi-onda/.pyenv/versions/3.12.7/lib/python3.12/site-packages/strands/multiagent/a2a/server.py", line 228, in serve
    uvicorn.run(self.to_starlette_app(), host=host or self.host, port=port or self.port, **kwargs)
                ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mi-onda/.pyenv/versions/3.12.7/lib/python3.12/site-packages/strands/multiagent/a2a/server.py", line 171, in to_starlette_app
    a2a_app = A2AStarletteApplication(agent_card=self.public_agent_card, http_handler=self.request_handler).build()
                                                 ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mi-onda/.pyenv/versions/3.12.7/lib/python3.12/site-packages/strands/multiagent/a2a/server.py", line 123, in public_agent_card
    return AgentCard(
           ^^^^^^^^^^
  File "/Users/mi-onda/.pyenv/versions/3.12.7/lib/python3.12/site-packages/pydantic/main.py", line 253, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 2 validation errors for AgentCard
defaultInputModes
  Field required [type=missing, input_value={'name': '俳句エージ...y=None, streaming=True)}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
defaultOutputModes
  Field required [type=missing, input_value={'name': '俳句エージ...y=None, streaming=True)}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
```

### Additional Context

a2a-protocol                             0.1.0
a2a-sdk                                  0.2.16
python-a2a                               0.5.9

### Possible Solution

_No response_

### Related Issues

_No response_

Labels: bug
State: closed