[Bug] ASGI cookie conversion serializes an absent Domain attribute and breaks __Host- cookies

## Expected Behavior

This surfaced in [FastMCP issue #4748](https://github.com/PrefectHQ/fastmcp/issues/4748). FastMCP's OAuth consent flow emits a valid `__Host-` cookie, but when the application runs on Azure Functions, the browser receives a `Domain` attribute and rejects it.

Azure Functions should preserve the absence of `Domain` in this ASGI response:

```http
Set-Cookie: __Host-test=value; Path=/; Secure; HttpOnly; SameSite=Lax
```

`__Host-` cookies require `Secure`, `Path=/`, and no `Domain` attribute.

## Actual Behavior

The Python ASGI response path parses `Set-Cookie` into a structured cookie and serializes the missing domain as a present, empty domain. The final response contains `domain=` or surfaces the Function App hostname as its effective domain. Browsers therefore reject the `__Host-` cookie; in FastMCP this causes the consent POST's CSRF check to fail.

## Steps to Reproduce

1. Deploy the ASGI application below to Azure Functions.
2. Request its HTTP endpoint over HTTPS.
3. Inspect the final `Set-Cookie` header and the browser cookie warnings.
4. Observe that the response includes a Domain attribute and the browser rejects `__Host-test`.

## Relevant code being tried

```python
import azure.functions as func

async def asgi_app(scope, receive, send):
assert scope["type"] == "http"

await send(
{
"type": "http.response.start",
"status": 200,
"headers": [
(b"content-type", b"text/plain"),
(
b"set-cookie",
b"__Host-test=value; Path=/; Secure; HttpOnly; SameSite=Lax",
),
],
}
)
await send(
{
"type": "http.response.body",
"body": b"ok",
"more_body": False,
}
)

app = func.AsgiFunctionApp(
app=asgi_app,
http_auth_level=func.AuthLevel.ANONYMOUS,
)
```

## Relevant log output

No application error is logged. The failure appears in the response header and browser cookie warning.

## requirements.txt file

```text
azure-functions==1.24.0
```

## Where are you facing this problem?

Production Environment

## Function app name

Not publicly shareable

## Additional Information

`azure-functions-python-library` removes the raw header and parses it with `SimpleCookie`:
https://github.com/Azure/azure-functions-python-library/blob/dev/azure/functions/http.py#L113-L116

The worker then passes the empty `cookie_entity['domain']` value to `to_nullable_string`, creating a present RPC domain:
https://github.com/Azure/azure-functions-python-worker/blob/dev/workers/azure_functions_worker/bindings/datumdef.py#L232-L245

The existing end-to-end test expects an attribute-free cookie to become `foo=bar; domain=; path=`:
https://github.com/Azure/azure-functions-python-worker/blob/dev/workers/tests/unittests/test_http_functions.py#L374-L379

A targeted fix would omit the RPC domain when `cookie_entity['domain']` is empty while preserving explicit domains. A `__Host-` regression test can verify that the final response contains `Path=/; Secure` and no `Domain`.

从 workers/azure_functions_worker/bindings/datumdef.py 第 232-245 行附近开始，然后运行 workers/tests/unittests/test_http_functions.py 第 374-379 行附近的相关测试。为提供的 __Host-test 响应添加一个回归用例，并验证缺失的 Domain 属性会被省略，而显式指定的域仍会被序列化。

由索引模型根据 Issue 内容生成。