feat(router): add order-based fallback so higher order deployments are tried on failure

When a request to an order=1 deployment fails, the router now
automatically tries order=2, order=3, etc. before falling through to
external fallbacks. Works for all error types (429, 404, connection
errors). Requires enable_pre_call_checks=True.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/my-website/docs/proxy/load_balancing.md

@@ -325,8 +325,15 @@ model_list:
       model: azure/gpt-4-fallback
       api_key: os.environ/AZURE_API_KEY_2
       order: 2  # 👈 Used when order=1 fails
+
+router_settings:
+  enable_pre_call_checks: true  # 👈 Required for 'order' to work
 ```
 
+:::important
+The `order` parameter requires `enable_pre_call_checks: true` in `router_settings`.
+:::
+
 ### How order-based fallback works
 
 When a request to an `order=1` deployment fails (connection error, 404, 429, etc.), the router automatically tries `order=2` deployments, then `order=3`, and so on. Each order level gets its own set of retries before escalating to the next.
@@ -353,6 +360,7 @@ model_list:
       api_key: os.environ/OPENAI_API_KEY
 
 router_settings:
+  enable_pre_call_checks: true
   fallbacks:
     - gpt-4:
         - gpt-4-fallback  # tried after all order levels fail

docs/my-website/docs/routing.md

@@ -869,9 +869,13 @@ model_list = [
     },
 ]
 
-router = Router(model_list=model_list)
+router = Router(model_list=model_list, enable_pre_call_checks=True)  # 👈 Required for 'order' to work
 ```
 
+:::important
+The `order` parameter requires `enable_pre_call_checks=True` to be set on the Router.
+:::
+
 </TabItem>
 <TabItem value="proxy" label="PROXY">
 
@@ -888,6 +892,9 @@ model_list:
       model: azure/gpt-4-fallback
       api_key: os.environ/AZURE_API_KEY_2
       order: 2  # 👈 Tried when order=1 fails
+
+router_settings:
+  enable_pre_call_checks: true  # 👈 Required for 'order' to work
 ```
 
 </TabItem>