test: prove SERVER_VERSION restoration independently of the list that performs it #18

Follow-up from the release-automation review (round 25 N1). Non-gating: the
environment behaviour is correct today; the observer is the weak part.

tests/transport/http.test.ts saves and restores SERVER_VERSION through
PINNED_ENV, and its teardown postcondition iterates that same list. So the
list is both the mechanism and its only witness: delete the key from it and you
delete the behaviour and the check that would have caught it, together. The
suite stays green.

This is the same shape as the other findings on that branch -- a check that
cannot fail for the reason it exists -- just at the fixture layer rather than in
the release path.

Fix (from the review): capture the inherited SERVER_VERSION in an independent
constant and assert its final presence/value directly, outside the PINNED_ENV
loop; or run this lifecycle in a subprocess with a known inherited value.

Verify by mutation: removing SERVER_VERSION from PINNED_ENV must make the
suite red. Today it does not.

Context: reviews/2026-07-17-release-automation-codex-review-r25.md