fix(test): remove free tier from GPT-5.5 inclusion test

GPT-5.5 was correctly removed from codex-free tier in 7b89583c
(since free accounts cannot access it), but the test was not updated
to reflect this. This caused TestCodexStaticModelsIncludeGPT55 to
fail on the free subtest.

Changes:
- Remove free tier from GPT-5.5 inclusion test
- Add new TestCodexFreeModelsExcludeGPT55 to explicitly verify
  that free tier does NOT include GPT-5.5

internal/registry/model_definitions_test.go

@@ -2,9 +2,15 @@ package registry
 
 import "testing"
 
+func TestCodexFreeModelsExcludeGPT55(t *testing.T) {
+	model := findModelInfo(GetCodexFreeModels(), "gpt-5.5")
+	if model != nil {
+		t.Fatal("expected codex free tier to NOT include gpt-5.5")
+	}
+}
+
 func TestCodexStaticModelsIncludeGPT55(t *testing.T) {
 	tierModels := map[string][]*ModelInfo{
-		"free": GetCodexFreeModels(),
 		"team": GetCodexTeamModels(),
 		"plus": GetCodexPlusModels(),
 		"pro":  GetCodexProModels(),