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_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 {