Score the boundary against wrong credentials, not just right ones (V-673)

Every shape of wrong credential gets a case: no header, wrong token, a prefix
of the token, the token with no scheme, and Basic. Plus the two the allowlist
exists for, /slots and its save action, and the caps.

The readiness test now posts to /v1/chat/completions. The allowlist sits in
front of the readiness check and answers 405 to a method mavgpud never
serves, so the old GET measured the allowlist rather than the 503.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ESv8hqNPseYt1CnotZpqDz

cmd/mavgpud/gpu_test.go

@@ -104,9 +104,11 @@ func TestHealthAndProxyRefuseWhenNotReady(t *testing.T) {
 	s := &supervisor{run: newRunner("fake", "/bin/true", nil, "")}
 	h := s.handler(mustURL(t, "http://127.0.0.1:1"))
 
-	for _, path := range []string{"/health", "/v1/chat/completions"} {
+	// The completion is a POST because the allowlist is in front of the
+	// readiness check now, and it answers 405 to a method it never serves.
+	for path, method := range map[string]string{"/health": http.MethodGet, "/v1/chat/completions": http.MethodPost} {
 		w := httptest.NewRecorder()
-		h.ServeHTTP(w, httptest.NewRequest(http.MethodGet, path, nil))
+		h.ServeHTTP(w, httptest.NewRequest(method, path, nil))
 		if w.Code != http.StatusServiceUnavailable {
 			t.Errorf("%s with no model: got %d, want 503", path, w.Code)
 		}