Bug: --enable-http-cache causes 304 Not Modified HIT traffic to drop to 0

### Description After enabling the advanced HTTP cache via the `--enable-http-cache` flag, we observed an immediate and total drop in our `304 Not Modified` HIT traffic. When the flag is turned off, standard 304 behavior (both HIT and PASS) instantly returns. This behavior is fully reproducible across our deployment boundaries. ### Expected Behavior Enabling the HTTP cache should respect conditional request headers (such as `If-None-Match` and `If-Modified-Since`) and properly synthesize or pass through 304 Not Modified responses. ### Actual Behavior 304 HIT traffic drops to near zero. Requests that should be served as a 304 are instead served as 200 OK HITs, or conditional revalidation fails entirely. ### Production Evidence (Fastly Compute) We monitored this behavior over a 5 day window. - With flag ON: HIT 304 traffic dropped to near 0 across all routes. - With flag OFF: HIT 304 traffic immediately returned to baseline levels. Correlations: The traffic shifts align perfectly with our deployment boundaries. ### Environment Details Platform: Fastly Compute Feature: --enable-http-cache flag this was originally discussed [here](https://github.com/fastly/js-compute-runtime/issues/1384#issuecomment-4389366943)