feat(xai): add Grok OAuth presets for Claude and Claude Desktop

Register the xai_oauth provider type constant and add xAI (Grok) presets
targeting grok-4.5. The Desktop preset routes branded claude-* model IDs
to grok-4.5 upstream so it passes Desktop's third-party model validation.

tests/config/xaiOauthProviderPresets.test.ts

@@ -0,0 +1,51 @@
+import { describe, expect, it } from "vitest";
+import { claudeDesktopProviderPresets } from "@/config/claudeDesktopProviderPresets";
+import { providerPresets } from "@/config/claudeProviderPresets";
+
+describe("xAI OAuth provider presets", () => {
+  it("pins the Claude Code preset to managed Responses auth", () => {
+    const preset = providerPresets.find((entry) => entry.name === "xAI (Grok)");
+    expect(preset).toBeDefined();
+    expect(preset).toMatchObject({
+      category: "third_party",
+      apiFormat: "openai_responses",
+      providerType: "xai_oauth",
+      requiresOAuth: true,
+      icon: "xai",
+    });
+    expect((preset!.settingsConfig as any).env).toMatchObject({
+      ANTHROPIC_BASE_URL: "https://api.x.ai/v1",
+      ANTHROPIC_MODEL: "grok-4.5",
+      ANTHROPIC_DEFAULT_HAIKU_MODEL: "grok-4.5",
+      ANTHROPIC_DEFAULT_SONNET_MODEL: "grok-4.5",
+      ANTHROPIC_DEFAULT_OPUS_MODEL: "grok-4.5",
+    });
+    expect((preset!.settingsConfig as any).env).not.toHaveProperty(
+      "ANTHROPIC_API_KEY",
+    );
+    expect((preset!.settingsConfig as any).env).not.toHaveProperty(
+      "ANTHROPIC_AUTH_TOKEN",
+    );
+  });
+
+  it("pins the Claude Desktop preset to proxy Responses mode without 1M", () => {
+    const preset = claudeDesktopProviderPresets.find(
+      (entry) => entry.name === "xAI (Grok)",
+    );
+    expect(preset).toMatchObject({
+      category: "third_party",
+      baseUrl: "https://api.x.ai/v1",
+      mode: "proxy",
+      apiFormat: "openai_responses",
+      providerType: "xai_oauth",
+      requiresOAuth: true,
+      icon: "xai",
+    });
+    expect(preset!.modelRoutes).toEqual([
+      expect.objectContaining({
+        upstreamModel: "grok-4.5",
+        supports1m: false,
+      }),
+    ]);
+  });
+});