feat(providers): add four OpenAI-compatible aggregator providers (dahl, atria, agnes, bai)

Adds Dahl Inference, Atria Dawn, Agnes AI, and B.AI following the existing
registry pattern. Each uses DefaultExecutor with no custom translator needed.

open-sse/providers/registry/bai.js

@@ -0,0 +1,30 @@
+export default {
+  id: "bai",
+  priority: 120,
+  alias: "bai",
+  aliases: [
+    "b-ai",
+  ],
+  uiAlias: "bai",
+  display: {
+    name: "B.AI",
+    icon: "account_balance",
+    color: "#0369A1",
+    textIcon: "BA",
+    website: "https://b.ai",
+    notice: {
+      text: "OpenAI-compatible gateway with one of the larger catalogues here. Accepts a bearer token or an x-api-key header. Model ids are fetched live from the provider.",
+      apiKeyUrl: "https://b.ai",
+    },
+  },
+  category: "apikey",
+  authType: "apikey",
+  transport: {
+    baseUrl: "https://api.b.ai/v1/chat/completions",
+    validateUrl: "https://api.b.ai/v1/models",
+  },
+  // No ids hardcoded: the catalogue is large and rotates, so the live endpoint
+  // is the source of truth and any id is accepted via passthroughModels.
+  modelsFetcher: { url: "https://api.b.ai/v1/models", type: "openai" },
+  passthroughModels: true,
+};