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.

tests/__baseline__/providers-baseline.json

@@ -1085,5 +1085,25 @@
       "preserveCacheControl": true
     },
     "format": "openai"
+  },
+  "dahl": {
+    "baseUrl": "https://inference.dahl.global/v1/chat/completions",
+    "validateUrl": "https://inference.dahl.global/v1/models",
+    "format": "openai"
+  },
+  "atria": {
+    "baseUrl": "https://api.atria-asi.ai/v1/chat/completions",
+    "validateUrl": "https://api.atria-asi.ai/v1/models",
+    "format": "openai"
+  },
+  "agnes": {
+    "baseUrl": "https://apihub.agnes-ai.com/v1/chat/completions",
+    "validateUrl": "https://apihub.agnes-ai.com/v1/models",
+    "format": "openai"
+  },
+  "bai": {
+    "baseUrl": "https://api.b.ai/v1/chat/completions",
+    "validateUrl": "https://api.b.ai/v1/models",
+    "format": "openai"
   }
 }