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.go

@@ -10,6 +10,7 @@ const (
 	codexBuiltinImageModelID      = "gpt-image-2"
 	xaiBuiltinImageModelID        = "grok-imagine-image"
 	xaiBuiltinImageQualityModelID = "grok-imagine-image-quality"
+	xaiBuiltinVideoModelID        = "grok-imagine-video"
 )
 
 // staticModelsJSON mirrors the top-level structure of models.json.
@@ -95,10 +96,10 @@ func WithCodexBuiltins(models []*ModelInfo) []*ModelInfo {
 	return upsertModelInfos(models, codexBuiltinImageModelInfo())
 }
 
-// WithXAIBuiltins injects hard-coded xAI image model definitions that should
+// WithXAIBuiltins injects hard-coded xAI image/video model definitions that should
 // not depend on remote models.json updates.
 func WithXAIBuiltins(models []*ModelInfo) []*ModelInfo {
-	return upsertModelInfos(models, xaiBuiltinImageModelInfo(), xaiBuiltinImageQualityModelInfo())
+	return upsertModelInfos(models, xaiBuiltinImageModelInfo(), xaiBuiltinImageQualityModelInfo(), xaiBuiltinVideoModelInfo())
 }
 
 func codexBuiltinImageModelInfo() *ModelInfo {
@@ -139,6 +140,19 @@ func xaiBuiltinImageQualityModelInfo() *ModelInfo {
 	}
 }
 
+func xaiBuiltinVideoModelInfo() *ModelInfo {
+	return &ModelInfo{
+		ID:          xaiBuiltinVideoModelID,
+		Object:      "model",
+		Created:     1735689600, // 2025-01-01
+		OwnedBy:     "xai",
+		Type:        "xai",
+		DisplayName: "Grok Imagine Video",
+		Name:        xaiBuiltinVideoModelID,
+		Description: "xAI Grok video generation model.",
+	}
+}
+
 func upsertModelInfos(models []*ModelInfo, extras ...*ModelInfo) []*ModelInfo {
 	if len(extras) == 0 {
 		return models

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 {