fix(ci): make the credential scrub failure-safe, and stop the lists drifting

Round 5, and the best findings are again about the fix from round 4.

The scrub only ran on success. `set -e` aborts the step when `go mod download`
fails, so the cleanup line after it never executed — leaving a push-capable
credential on a long-lived self-hosted runner for whatever job landed there
next. It is now a `trap ... EXIT`, verified against a simulated failure.

It also scrubbed the wrong file in principle: `git config --global` writes to
GIT_CONFIG_GLOBAL, else $XDG_CONFIG_HOME/git/config when that exists, else
~/.gitconfig — so deleting ~/.gitconfig can scrub a path the credential was
never in. The step now names GIT_CONFIG_GLOBAL itself, leaving exactly one file
to remove.

And the verification failed open in the case that matters most: `grep -F ""`
matches every file, so a run WITHOUT the secret — a fork PR, the threat model —
failed the check with a message accusing it of leaking a credential it never
had. Guarded on a non-empty secret.

Credentials move to an Authorization header instead of being embedded in the
URL, so a password containing @ : / or # can no longer break URL parsing in a
way that reads as a bad password.

Two list-drift holes closed with one test that reads across languages:
TestOpenAICompatProvidersAreFullyWired asserts every openAICompatProviders
entry is both advertised in endpointProviderNames and has a credential arm in
scripts/preflight.sh. Adding a compat provider touches three places in two
languages and nothing connected them. Break-checked in both directions.

Finally, a whitespace-only GADFLY_BASE_URL disagreed across the boundary: Go
TrimSpaces it and takes the registry path, bash called it "set" and skipped the
pre-flight, so the missing key arrived as a bare 401 with no notice. Both now
agree on what unset means.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

scripts/preflight_test.sh

@@ -79,6 +79,11 @@ done
 # GEMINI_API_KEY, so a one-name arm would skip a correctly-configured run.
 check "google w/ only GEMINI_API_KEY" "" "$(probe google GEMINI_API_KEY=k)"
 
+echo "== a whitespace-only GADFLY_BASE_URL counts as unset, as it does in Go =="
+# resolveModel TrimSpaces it and takes the registry path; if this check
+# disagreed, the missing key would arrive as a bare 401 with no skip notice.
+check "qwen + blank BASE_URL" "QWEN_API_KEY" "$(probe qwen GADFLY_BASE_URL="   ")"
+
 if [ "$fail" -ne 0 ]; then
   echo "RESULT: preflight table FAILED"
   exit 1