feat(runtime): track upstream response headers in logging and usage reporting

- Added APIs to store, retrieve, and clone upstream response headers in context for detailed logging.
- Updated `RecordAPIResponseMetadata`, `RecordAPIWebsocketHandshake`, and related methods to capture response headers.
- Extended `UsageReporter` to include response headers in published usage records.
- Enhanced payload tests to validate response headers' integrity and persistence.
- Refactored `usage.Record` to support optional `ResponseHeaders` field.

internal/runtime/executor/helps/logging_helpers_test.go

@@ -0,0 +1,24 @@
+package helps
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/router-for-me/CLIProxyAPI/v7/internal/config"
+	"github.com/router-for-me/CLIProxyAPI/v7/internal/logging"
+)
+
+func TestRecordAPIResponseMetadataStoresHeadersWhenRequestLogDisabled(t *testing.T) {
+	ctx := logging.WithResponseHeadersHolder(context.Background())
+	headers := http.Header{}
+	headers.Add("X-Upstream-Request-Id", "upstream-req-1")
+
+	RecordAPIResponseMetadata(ctx, &config.Config{}, http.StatusOK, headers)
+	headers.Set("X-Upstream-Request-Id", "mutated")
+
+	got := logging.GetResponseHeaders(ctx)
+	if got.Get("X-Upstream-Request-Id") != "upstream-req-1" {
+		t.Fatalf("response header = %q, want %q", got.Get("X-Upstream-Request-Id"), "upstream-req-1")
+	}
+}