[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`.

workers/azure_functions_worker/bindings/datumdef.py の232-245行付近から始め、続いて workers/tests/unittests/test_http_functions.py の374-379行付近にある関連テストを実行します。提供された __Host-test レスポンスの回帰ケースを追加し、存在しない Domain 属性が省略される一方で、明示的なドメインは引き続きシリアライズされることを確認します。

索引モデルが issue の本文から書いたものです。