Upgrades yesterday's #99310 skip-guard to full pointer-carry from
PR #90484: model assignment and custom-endpoint activation now write
key_env or the raw ${VAR} template into model config instead of
dropping the credential reference entirely, so the model entry keeps
resolving at runtime with zero plaintext in config.yaml. Applied
surgically onto current main (the PR branch predates newer
web_server.py changes); key_env carry made independent of the
expanded api_key guard, tests updated to pin pointer-carry.
teknium1