Fix: JSON RPC format in A2AClient to connect with third party a2a servers

[0xrushi]

Before

=== Send Messages to the Agent ===
Type your messages (or 'exit' to quit):

> hi

Sending message to agent...

Agent response:
{"error":{"code":-32600,"data":[{"type":"missing","loc":["SendMessageRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"},{"type":"missing","loc":["SendStreamingMessageRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"},{"type":"missing","loc":["GetTaskRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"},{"type":"missing","loc":["CancelTaskRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"},{"type":"missing","loc":["SetTaskPushNotificationConfigRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"},{"type":"missing","loc":["GetTaskPushNotificationConfigRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"},{"type":"missing","loc":["TaskResubscriptionRequest","params"],"msg":"Field required","input":{"content":{"text":"hi","type":"text"},"role":"user","message_id":"bfafb0c0-56a2-415e-957d-b3765debe760"},"url":"https://errors.pydantic.dev/2.11/v/missing"}],"message":"Request payload validation error"},"jsonrpc":"2.0"}

> 

After

$ uv run simple_client_thirdparty_googlea2a_server.py --external  http://0.0.0.0:8080
✅ All dependencies are installed correctly!

🚀 Connecting to external A2A agent at: http://0.0.0.0:8080
🔌 Connecting to A2A agent at: http://0.0.0.0:8080

=== Agent Information ===
Name: Unknown Agent
Description: Agent card not available
Version: unknown

=== Send Messages to the Agent ===
Type your messages (or 'exit' to quit):

> hi

Sending message to agent...

Agent response:
{"id":1,"jsonrpc":"2.0","result":{"kind":"message","messageId":"8fff72fc-101a-44e0-99ed-92a822b0799c","parts":[{"kind":"text","text":"Hello World"}],"role":"agent"}}

> 

You can try connecting to any of the servers from google a2a repo
https://github.com/google/a2a-python/tree/main/examples

[0xrushi]

resolves #56