fix(handlers): include execution session metadata and skip idempotency key when absent

- Refactored `requestExecutionMetadata` to handle empty `Idempotency-Key` gracefully.
- Added test to validate metadata inclusion of execution session without idempotency key.

sdk/api/handlers/handlers_metadata_test.go

@@ -0,0 +1,20 @@
+package handlers
+
+import (
+	"testing"
+
+	coreexecutor "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
+	"golang.org/x/net/context"
+)
+
+func TestRequestExecutionMetadataIncludesExecutionSessionWithoutIdempotencyKey(t *testing.T) {
+	ctx := WithExecutionSessionID(context.Background(), "session-1")
+
+	meta := requestExecutionMetadata(ctx)
+	if got := meta[coreexecutor.ExecutionSessionMetadataKey]; got != "session-1" {
+		t.Fatalf("ExecutionSessionMetadataKey = %v, want %q", got, "session-1")
+	}
+	if _, ok := meta[idempotencyKeyMetadataKey]; ok {
+		t.Fatalf("unexpected idempotency key in metadata: %v", meta[idempotencyKeyMetadataKey])
+	}
+}