feat(auth): add OAuth2 support for xAI with PKCE and token persistence

- Implemented xAI OAuth2 integration with PKCE (Proof Key for Code Exchange) support.
- Added logic for token exchange, refresh, and persistent storage in JSON format.
- Created `xai` package with helpers for OAuth discovery, API token handling, and URL building.
- Introduced `XAIExecutor` for integrating xAI credentials into runtime HTTP requests.
- Added unit tests to validate OAuth flow, token persistence, and endpoint validation.

internal/registry/model_definitions.go

@@ -21,6 +21,7 @@ type staticModelsJSON struct {
 	CodexPro    []*ModelInfo `json:"codex-pro"`
 	Kimi        []*ModelInfo `json:"kimi"`
 	Antigravity []*ModelInfo `json:"antigravity"`
+	XAI         []*ModelInfo `json:"xai"`
 }
 
 // GetClaudeModels returns the standard Claude model definitions.
@@ -78,6 +79,11 @@ func GetAntigravityModels() []*ModelInfo {
 	return cloneModelInfos(getModels().Antigravity)
 }
 
+// GetXAIModels returns the standard xAI Grok model definitions.
+func GetXAIModels() []*ModelInfo {
+	return cloneModelInfos(getModels().XAI)
+}
+
 // WithCodexBuiltins injects hard-coded Codex-only model definitions that should
 // not depend on remote models.json updates. Built-ins replace any matching IDs
 // already present in the provided slice.
@@ -167,6 +173,7 @@ func cloneModelInfos(models []*ModelInfo) []*ModelInfo {
 //   - codex
 //   - kimi
 //   - antigravity
+//   - xai
 func GetStaticModelDefinitionsByChannel(channel string) []*ModelInfo {
 	key := strings.ToLower(strings.TrimSpace(channel))
 	switch key {
@@ -186,6 +193,8 @@ func GetStaticModelDefinitionsByChannel(channel string) []*ModelInfo {
 		return GetKimiModels()
 	case "antigravity":
 		return GetAntigravityModels()
+	case "xai", "x-ai", "grok":
+		return GetXAIModels()
 	default:
 		return nil
 	}
@@ -208,6 +217,7 @@ func LookupStaticModelInfo(modelID string) *ModelInfo {
 		data.CodexPro,
 		data.Kimi,
 		data.Antigravity,
+		data.XAI,
 	}
 	for _, models := range allModels {
 		for _, m := range models {

internal/registry/model_updater.go

@@ -215,6 +215,7 @@ func detectChangedProviders(oldData, newData *staticModelsJSON) []string {
 		{"codex", oldData.CodexPro, newData.CodexPro},
 		{"kimi", oldData.Kimi, newData.Kimi},
 		{"antigravity", oldData.Antigravity, newData.Antigravity},
+		{"xai", oldData.XAI, newData.XAI},
 	}
 
 	seen := make(map[string]bool, len(sections))
@@ -335,6 +336,7 @@ func validateModelsCatalog(data *staticModelsJSON) error {
 		{name: "codex-pro", models: data.CodexPro},
 		{name: "kimi", models: data.Kimi},
 		{name: "antigravity", models: data.Antigravity},
+		{name: "xai", models: data.XAI},
 	}
 
 	for _, section := range requiredSections {

internal/registry/models/models.json

@@ -46,7 +46,8 @@
         "levels": [
           "low",
           "medium",
-          "high"
+          "high",
+          "xhigh"
         ]
       }
     },
@@ -2064,5 +2065,109 @@
         ]
       }
     }
+  ],
+  "xai": [
+    {
+      "id": "grok-4.3",
+      "object": "model",
+      "created": 1775606400,
+      "owned_by": "xai",
+      "type": "xai",
+      "display_name": "Grok 4.3",
+      "name": "grok-4.3",
+      "description": "xAI Grok 4.3 model for the Responses API.",
+      "context_length": 1000000,
+      "max_completion_tokens": 65536,
+      "thinking": {
+        "zero_allowed": true,
+        "levels": [
+          "none",
+          "low",
+          "medium",
+          "high"
+        ]
+      }
+    },
+    {
+      "id": "grok-4.20-0309-reasoning",
+      "object": "model",
+      "created": 1773014400,
+      "owned_by": "xai",
+      "type": "xai",
+      "display_name": "Grok 4.20 0309 Reasoning",
+      "name": "grok-4.20-0309-reasoning",
+      "description": "xAI Grok 4.20 0309 reasoning model for the Responses API.",
+      "context_length": 2000000,
+      "max_completion_tokens": 65536
+    },
+    {
+      "id": "grok-4.20-0309-non-reasoning",
+      "object": "model",
+      "created": 1773014400,
+      "owned_by": "xai",
+      "type": "xai",
+      "display_name": "Grok 4.20 0309 Non Reasoning",
+      "name": "grok-4.20-0309-non-reasoning",
+      "description": "xAI Grok 4.20 0309 non-reasoning model for the Responses API.",
+      "context_length": 2000000,
+      "max_completion_tokens": 65536
+    },
+    {
+      "id": "grok-4.20-multi-agent-0309",
+      "object": "model",
+      "created": 1773014400,
+      "owned_by": "xai",
+      "type": "xai",
+      "display_name": "Grok 4.20 Multi Agent 0309",
+      "name": "grok-4.20-multi-agent-0309",
+      "description": "xAI Grok 4.20 multi-agent model for the Responses API.",
+      "context_length": 2000000,
+      "max_completion_tokens": 65536,
+      "thinking": {
+        "levels": [
+          "low",
+          "medium",
+          "high"
+        ]
+      }
+    },
+    {
+      "id": "grok-3-mini",
+      "object": "model",
+      "created": 1740960000,
+      "owned_by": "xai",
+      "type": "xai",
+      "display_name": "Grok 3 Mini",
+      "name": "grok-3-mini",
+      "description": "xAI Grok 3 Mini model for the Responses API.",
+      "context_length": 131072,
+      "max_completion_tokens": 32768,
+      "thinking": {
+        "levels": [
+          "low",
+          "medium",
+          "high"
+        ]
+      }
+    },
+    {
+      "id": "grok-3-mini-fast",
+      "object": "model",
+      "created": 1740960000,
+      "owned_by": "xai",
+      "type": "xai",
+      "display_name": "Grok 3 Mini Fast",
+      "name": "grok-3-mini-fast",
+      "description": "xAI Grok 3 Mini Fast model for the Responses API.",
+      "context_length": 131072,
+      "max_completion_tokens": 32768,
+      "thinking": {
+        "levels": [
+          "low",
+          "medium",
+          "high"
+        ]
+      }
+    }
   ]
 }