feat(api, xai): add xAI Grok video model support with API integration

- Introduced new xAI `grok-imagine-video` model for video generation with configurable options (e.g., duration, size, resolution).
- Implemented video-specific API endpoints (`/v1/videos`, `/v1/videos/generations`, `/v1/videos/edits`, `/v1/videos/extensions`), including request validation and model handling.
- Enhanced model registry with `xaiBuiltinVideoModelID` and metadata for video capabilities.
- Added unit tests to validate video model support, request structures, and API response handling.
- Extended `XAIExecutor` to integrate video generation and retrieval via runtime requests.

internal/registry/model_definitions_test.go

@@ -33,6 +33,22 @@ func TestCodexStaticModelsIncludeGPT55(t *testing.T) {
 	assertGPT55ModelInfo(t, "lookup", model)
 }
 
+func TestWithXAIBuiltinsAddsVideoModel(t *testing.T) {
+	models := WithXAIBuiltins(nil)
+	found := false
+	for _, model := range models {
+		if model != nil && model.ID == xaiBuiltinVideoModelID {
+			found = true
+			if model.OwnedBy != "xai" {
+				t.Fatalf("OwnedBy = %q, want xai", model.OwnedBy)
+			}
+		}
+	}
+	if !found {
+		t.Fatalf("expected %s builtin model", xaiBuiltinVideoModelID)
+	}
+}
+
 func findModelInfo(models []*ModelInfo, id string) *ModelInfo {
 	for _, model := range models {
 		if model != nil && model.ID == id {