Improvments to fuzz_coverage

Based on the conersation in https://github.com/envoyproxy/envoy/pull/39030 (see https://github.com/envoyproxy/envoy/pull/39030#issuecomment-2807912729, https://github.com/envoyproxy/envoy/pull/39030#issuecomment-2819632230 and https://github.com/envoyproxy/envoy/pull/39030#issuecomment-2821313281) it looks like `fuzz_coverage` CI target does not quite measure what we want it to measure (we want it to measure what coverage the fuzzing achieves, while what it currently measures is coverage of the initial seed without what fuzzing on top of that achieves).

I did a brief search for `fuzz_coverage` related open issues in the repository, but I didn't find anything open already. I'm opening this issue to consider how we can measure the covearage achieved by fuzzing (or conclude that we can't do that for some reason).

NOTE: At the moment I don't have any good suggestions on how we can actually make `fuzz_coverage` target more useful, since I don't have full understanding how it works, so I'm keeping this issue a bit open ended.

@phlax @yanavlasov @adisuissa