feat: add xai media providers

Add xAI image generation and text-to-speech provider support with docs, live tests, and guarded provider HTTP handling.\n\nThanks @KateWilkins.

test/image-generation.runtime.live.test.ts

@@ -79,6 +79,11 @@ const PROVIDER_CASES: LiveProviderCase[] = [
     pluginName: "Vydra Provider",
     providerId: "vydra",
   },
+  {
+    pluginId: "xai",
+    pluginName: "xAI Provider",
+    providerId: "xai",
+  },
 ]
   .filter((entry) => (providerFilter ? providerFilter.has(entry.providerId) : true))
   .toSorted((left, right) => left.providerId.localeCompare(right.providerId));