Add support for the HTTP HEAD method in StaticFileHandler

[kappsegla]

The server currently supports only the GET method. To behave as a standards‑compliant HTTP server, it should also implement the HEAD method according to RFC 9110.

Expected behavior

• A HEAD request returns the same status code and headers as the corresponding GET request but without a message body.
• Headers such as Content-Length, Content-Type, and caching headers should still be included.
• Error responses (e.g., 404, 500) must follow the same rule: identical headers, no body.
• Connection handling should remain consistent with normal HTTP semantics.

Suggested implementation outline

• Reuse the existing GET handler to compute metadata (status, headers).
• Suppress the body before writing to the socket when the method is HEAD.
• Ensure routing logic treats HEAD as a first‑class method.
• Add automated tests verifying:
  • Identical headers between GET and HEAD.
  • No body is sent for HEAD.
  • Correct behavior for both success and error responses.

Motivation

Supporting HEAD improves standards compliance, enables efficient metadata checks, and ensures compatibility with proxies, crawlers, and other HTTP‑aware tooling.