feat: add httpx2 support - #1439

…onfig (#1445)

# Description

`RestTransport.delete_task_push_notification_config` goes through
`_execute_request`, which always calls `response.json()`. The spec
leaves the result to the server
([3.1.10](https://a2a-protocol.org/latest/specification/#3110-delete-push-notification-config):
"Confirmation of deletion (implementation-specific)"), and the SDKs
answer differently:

- this SDK's server answers 200 with `{}`;
- a2a-js answers 204 No Content
([`rest_handler.ts`](https://github.com/a2aproject/a2a-js/blob/3d3ca8291e1f6b1f53293f61c12ba46c36670000/src/server/express/rest_handler.ts#L548-L559));
- a2a-go answers 200 with no body
([`handleDeleteTaskPushConfig`](https://github.com/a2aproject/a2a-go/blob/ea739de3859f0574175bc9876eda39b7120da8e3/a2asrv/rest.go#L536-L557)).

Against a2a-js or a2a-go servers, the config is deleted, but the Python
client then raises `A2AClientError: JSON Decode Error`. A caller that
retries or reports the failure is acting on a delete that succeeded.

The delete now sends its request, checks the status, and does not read
the body. Error responses still go through `_handle_http_error`, so a
404 with `TASK_NOT_FOUND` still raises `TaskNotFoundError`.
`send_http_request` and the JSON-RPC transport are unchanged.

The repro at the end of this description answers the delete the way each
server does. On main (2cc1dd5):

```
a2a-python 200 b'{}' -> ok
a2a-js     204 b''   -> A2AClientError: JSON Decode Error: Expecting value: line 1 column 1 (char 0)
a2a-go     200 b''   -> A2AClientError: JSON Decode Error: Expecting value: line 1 column 1 (char 0)
```

On this branch, all three print `ok`.

```
$ pytest tests/client/transports/test_rest_client.py   # src change reverted
FAILED ...::TestTaskCallback::test_delete_task_push_notification_config_ignores_body[no_content]
FAILED ...::TestTaskCallback::test_delete_task_push_notification_config_ignores_body[empty_200]
2 failed, 44 passed
$ pytest --ignore=tests/integration/cross_version   # this branch, Python 3.10, locked deps
2560 passed, 207 skipped, 4 xfailed, 1 xpassed
$ pytest --cov=src
src/a2a/client/transports/rest.py   87%   (the new lines are covered)
TOTAL   94%
```

The new tests also cover a mapped error and a closed client on this
path. I left the cross-version tests out because their servers don't
start within the 3 s timeout on my machine; the same happens on main.
`ruff check`, `ruff format` and `ty check` pass. `ty` prints only the
three existing unused `type: ignore` warnings.

#1439 touches `send_http_request` and the transport type hints. This
change stays inside `delete_task_push_notification_config` to keep out
of its way, apart from one added import. #1436 percent-encodes the ids
in this method's path, so whichever of the two lands second has a
one-line conflict there; I'll rebase this one if #1436 goes first.

- [x] Follow the [`CONTRIBUTING`
Guide](https://github.com/a2aproject/a2a-python/blob/main/CONTRIBUTING.md).
- [x] Make your Pull Request title in the
<https://www.conventionalcommits.org/> specification.
- [x] Ensure the tests and linter pass (Run `bash scripts/format.sh`
from the repository root to format)
- [x] Appropriate docs were updated (if necessary)

No issue exists, since the new tests reproduce it.

<details><summary>Repro script</summary>

```python
import asyncio

import httpx

from a2a.client.transports.rest import RestTransport
from a2a.types.a2a_pb2 import AgentCard, DeleteTaskPushNotificationConfigRequest

# How each SDK's server confirms DELETE /tasks/{id}/pushNotificationConfigs/{configId}.
SERVERS = {'a2a-python': (200, b'{}'), 'a2a-js': (204, b''), 'a2a-go': (200, b'')}


async def main() -> None:
    for name, (status, body) in SERVERS.items():
        transport = httpx.MockTransport(lambda r, s=status, b=body: httpx.Response(s, content=b))
        async with httpx.AsyncClient(transport=transport) as http:
            client = RestTransport(http, AgentCard(), 'http://agent.example')
            try:
                await client.delete_task_push_notification_config(
                    DeleteTaskPushNotificationConfigRequest(task_id='t1', id='cfg1')
                )
                print(f'{name:10} {status} {body!r:5} -> ok')
            except Exception as e:
                print(f'{name:10} {status} {body!r:5} -> {type(e).__name__}: {e}')


asyncio.run(main())
```

</details>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <[email protected]>
Co-authored-by: mykytanetipa <[email protected]>