WebServer: Send HTML body with 301 redirects

[gogalvijay]

Previously, directory redirects (e.g., requesting /path without a trailing slash) sent a 301 Moved Permanently response consisting only of headers. This caused clients that expect a response body or connection closure (such as curl or strict HTTP/1.0 clients) to hang indefinitely waiting for content.

This patch adds a minimal HTML body to the 301 response, compliant with RFC 2616. This ensures clients can correctly detect the end of the message and close the connection immediately.

Testing:
Verified using curl and pro inside SerenityOS.

1. Created a test directory: mkdir /var/www/test_redirect
2. Requested without trailing slash: curl -v http://127.0.0.1:8000/test_redirect
3. Confirmed the request completes immediately and prints the HTML body.

Fixes the issue where 301 redirects served by the WebServer get stuck.

[BuggieBot]

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[LucasChollet]

It doesn't seem like we need to send a body with the redirect. Using python's python -m http.server I got this result with curl:

< HTTP/1.0 301 Moved Permanently
< Server: SimpleHTTP/0.6 Python/3.13.7
< Date: Wed, 24 Dec 2025 16:38:05 GMT
< Location: /Downloads/
< Content-Length: 0
< 
* shutting down connection #0

[LucasChollet]

Also, this is related to #26431. (if you add Fixes #ISSUE_NUMBER to the PR description, it will automatically close the issue when the PR is merged).