refactor(codex): remove stale affinity cleanup leftovers

Drop the last affinity-related executor artifacts so the PR stays focused on the minimal Codex continuity fix set: stable prompt cache identity, stable session_id, and the executor-only behavior that was validated to restore cache reads.

internal/runtime/executor/codex_continuity.go

@@ -14,8 +14,6 @@ import (
 	"github.com/tidwall/sjson"
 )
 
-const codexAuthAffinityMetadataKey = "auth_affinity_key"
-
 type codexContinuity struct {
 	Key    string
 	Source string

internal/runtime/executor/codex_executor_cache_test.go

@@ -178,18 +178,3 @@ func TestCodexExecutorCacheHelper_ClaudePreservesCacheContinuity(t *testing.T) {
 		t.Fatalf("session_id = %q, want %q", got, continuity.Key)
 	}
 }
-
-func TestResolveCodexContinuity_DoesNotForwardAuthAffinityKey(t *testing.T) {
-	req := cliproxyexecutor.Request{Payload: []byte(`{"model":"gpt-5.4"}`)}
-	opts := cliproxyexecutor.Options{Metadata: map[string]any{"auth_affinity_key": "principal:raw-client-secret"}}
-	auth := &cliproxyauth.Auth{ID: "codex-auth-1", Provider: "codex"}
-
-	continuity := resolveCodexContinuity(context.Background(), auth, req, opts)
-
-	if continuity.Source != "auth_id" {
-		t.Fatalf("continuity.Source = %q, want %q", continuity.Source, "auth_id")
-	}
-	if continuity.Key == "principal:raw-client-secret" {
-		t.Fatal("continuity.Key leaked raw auth affinity key")
-	}
-}