refactor: extract JSON-RPC specific logic from client (#169)

# Description

Currently `A2AClient` defined in `client.ts` uses JSON-RPC types in its
interface, i.e. `sendMessage` uses
[`SendMessageResponse`](https://github.com/a2aproject/a2a-js/blob/e7e8f35d5d5356d30a13131dea0caff97be8cd53/src/client/client.ts#L207)
which contains [JSON-RPC fields like `id` and `jsonrpc`
version](https://github.com/a2aproject/a2a-js/blob/e7e8f35d5d5356d30a13131dea0caff97be8cd53/src/types.ts#L2177-L2190).
It was also the case for Python SDK which required introducing a new
[client.py](https://github.com/a2aproject/a2a-python/blob/main/src/a2a/client/client.py)
keeping the old one as
[legacy.py](https://github.com/a2aproject/a2a-python/blob/main/src/a2a/client/legacy.py)
for backward compatibility (see
[#348](a2aproject/a2a-python#348)).

As a first step of introducing a transport-agnostic client, extract
JSON-RPC specific logic into a transport abstraction and switch existing
`A2AClient` to it in a backward compatible way to take advantage of
existing tests.

**Note:** new types are not exported from `index.ts`, this will be done
once new client is ready.

# Changes

1. Define `A2ATransport` abstraction without using JSON-RPC types.
2. Implement `JsonRpcTransport` from existing `A2AClient`.
3. Use new `JsonRpcTransport` from `A2AClient` - new transports are
**not going** to be added into this client as it uses JSON-RPC types.
This is done in a backward compatible way, existing behavior is
preserved: returning JSON-RPC errors as objects, populating JSON-RPC
specific fields.
4. Define shared transport-agnostic errors for A2A specific errors
defined in [the
spec](https://a2a-protocol.org/latest/specification/#82-a2a-specific-errors).

Re #137, #142

Author: ishymko