fix: A2A JSON-RPC v2: 1.0 Protocol-compliant

[ilyakorovin]

closes #4370

Fixed agent card

Now it's 1.0-compliant. Official Python SDK recognizes it automatically without explicit protocol version and agent card path specifying.

Fixed methods names

message/send -> SendMessage

task/get -> GetTask

Previous impl was for 0.3

Fixed client messageId

Previously it was effectively ignored.

Message parts are optional

parts field is required by A2A Protocol, but Python SDK converts empty parts array to undefined.

On our side message.parts are always defined.

Fixed potential losing history in stateful mode

I didn't catch the bug in live, but potentially in A2A JSON-RPC it was possible to get an error in approval decision request, because of stale data via pg update+select anti-pattern.

Some tests and small bug fixes

There were bugs like missing contextId and taskId in the SendMessage response.

---

There is a sample tool that uses Python SDK for working with Archestra A2A JSON-RPC v2:

https://github.com/ilyakorovin/archestra-a2a-client-sample/tree/main/python

It's LLM-generated and ugly. But it supports interactive mode:

cd ./python
uv run a2a-client-sample interactive --host localhost --port 9000 <AGENT_ID> <TOKEN>   

Just type some text and press enter, it will execute SendMessage. In case of approval request it should ask "y/n". Further messages during the session should be within the same contextId.

It also supports message/approve sub-commands for more manual working with SendMessage and approval decisions.