avoid unnecessary copy in cache code path

[tdyas]

[ci skip-build-wheels]

Problem

#11344 introduced a .to_bytes() helper for protobuf messages. However, in one spot, this introduced a unnecessary copy because the result is converted to a Vec<u8> immediately.

Solution

Remove the unnecessary copy by encoding into a Vec<u8> directly.

It might be possible to switch the PlatformAndResponseBytes struct to use Bytes instead of Vec<u8> but that would require reasoning about whether it would change the format of the struct when stored in the cache, which I want to avoid having to do for now.

Result

Existing tests pass.