[BUG] Adding BackgroundTask to Redirect results in 200 response. #728

Describe the bug
Adding BackgroundTask to Redirect response results in 200 status code.

Minimal Reproducible Example

app = FastHTML()
cli = TestClient(app, follow_redirects=False)

@app.get("/background")
def background_task():
    def long_running_task():
        time.sleep(0.1)
        print("Background task completed!")
    return Redirect("/"), BackgroundTask(long_running_task)

r = cli.get("/background")
print(r)
print(r.text)

Output:

Background task completed!
<Response [200 OK]>
 <!doctype html>
 <html>
   <head>
     <title>FastHTML page</title>
     <meta charset="utf-8">
...
   <body><fasthtml.core.Redirect object at 0x103863cb0></body>
 </html>

Expected behavior
Expect response. with 303 status code

Environment Information
Please provide the following version information:

• fastlite version: 0.1.3
• fastcore version: 1.8.1
• fasthtml version: 0.12.5

Confirmation
Please confirm the following:

• I have read the FAQ (https://docs.fastht.ml/explains/faq.html)
• I have provided a minimal reproducible example
• I have included the versions of fastlite, fastcore, and fasthtml
• I understand that this is a volunteer open source project with no commercial support.