feat(api, xai): integrate xAI Grok image models and extend API endpoints for image support

- Added new xAI Grok image models (`grok-imagine-image`, `grok-imagine-image-quality`) with high-fidelity and aspect ratio configurations.
- Extended `isSupportedImagesModel` logic to validate xAI models.
- Implemented API request builders for image generation/editing with customizable options (e.g., resolution, aspect ratio, response format).
- Enhanced `/v1/images` endpoints to handle xAI model capabilities, including response normalization and model-specific handlers.
- Updated unit tests to validate xAI model validation, request structure, and API integration.

internal/registry/model_definitions.go

@@ -6,7 +6,11 @@ import (
 	"strings"
 )
 
-const codexBuiltinImageModelID = "gpt-image-2"
+const (
+	codexBuiltinImageModelID      = "gpt-image-2"
+	xaiBuiltinImageModelID        = "grok-imagine-image"
+	xaiBuiltinImageQualityModelID = "grok-imagine-image-quality"
+)
 
 // staticModelsJSON mirrors the top-level structure of models.json.
 type staticModelsJSON struct {
@@ -81,7 +85,7 @@ func GetAntigravityModels() []*ModelInfo {
 
 // GetXAIModels returns the standard xAI Grok model definitions.
 func GetXAIModels() []*ModelInfo {
-	return cloneModelInfos(getModels().XAI)
+	return WithXAIBuiltins(cloneModelInfos(getModels().XAI))
 }
 
 // WithCodexBuiltins injects hard-coded Codex-only model definitions that should
@@ -91,6 +95,12 @@ func WithCodexBuiltins(models []*ModelInfo) []*ModelInfo {
 	return upsertModelInfos(models, codexBuiltinImageModelInfo())
 }
 
+// WithXAIBuiltins injects hard-coded xAI image model definitions that should
+// not depend on remote models.json updates.
+func WithXAIBuiltins(models []*ModelInfo) []*ModelInfo {
+	return upsertModelInfos(models, xaiBuiltinImageModelInfo(), xaiBuiltinImageQualityModelInfo())
+}
+
 func codexBuiltinImageModelInfo() *ModelInfo {
 	return &ModelInfo{
 		ID:          codexBuiltinImageModelID,
@@ -103,6 +113,32 @@ func codexBuiltinImageModelInfo() *ModelInfo {
 	}
 }
 
+func xaiBuiltinImageModelInfo() *ModelInfo {
+	return &ModelInfo{
+		ID:          xaiBuiltinImageModelID,
+		Object:      "model",
+		Created:     1735689600, // 2025-01-01
+		OwnedBy:     "xai",
+		Type:        "xai",
+		DisplayName: "Grok Imagine Image",
+		Name:        xaiBuiltinImageModelID,
+		Description: "xAI Grok image generation model.",
+	}
+}
+
+func xaiBuiltinImageQualityModelInfo() *ModelInfo {
+	return &ModelInfo{
+		ID:          xaiBuiltinImageQualityModelID,
+		Object:      "model",
+		Created:     1735689600, // 2025-01-01
+		OwnedBy:     "xai",
+		Type:        "xai",
+		DisplayName: "Grok Imagine Image Quality",
+		Name:        xaiBuiltinImageQualityModelID,
+		Description: "xAI Grok higher-fidelity image generation model.",
+	}
+}
+
 func upsertModelInfos(models []*ModelInfo, extras ...*ModelInfo) []*ModelInfo {
 	if len(extras) == 0 {
 		return models