Fix: return eTag with 304 responses

[Darilana]

This pull request adds an ETag to the response headers for 304 Not Modified responses, as specified by RFC 9110, Section 15.4.5

The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request:

• Content-Location, Date, ETag, and Vary

Including the ETag in these responses helps clients validate cached content correctly and reduces unnecessary data transfers.
This change brings the implementation more in line with HTTP standards.