New VSC counter c_fresh_invalid_len

BBlackrequested to merge
work/bblack/invalid_len into main

This distinguishes empty/missing cookie values in our statics (c_fresh_null) from defined cookie values that were not exactly 64 bytes long.

No external behavioral change here, just stats counter changes. It's unfortunately a bit clunky how we have to pass the "bad_len" indicator down through a few levels of function calls, but otherwise the logic and/or data structure changes would be more logically-invasive than I'd like at this stage.

Note there was already a testcase with the cookie value "whatever" in src/vtc/basic.vtc, so that test file was updated to note the new change in stats outcomes.

Bug: T394862

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