docs(router): replace .original_url with .default in /expressions (#7335)

Author: kamilkisiela

packages/web/docs/src/content/router/configuration/expressions.mdx

@@ -31,7 +31,7 @@ control‑flow. For example:
 if .request.headers."x-feature-toggle" == "beta" {
   "https://beta.example.com/graphql"
 } else {
-  .original_url
+  .default
 }
 ```
 
@@ -89,13 +89,29 @@ While not a full object, the special variable `.timestamp` yields the current UN
 header manipulation guide demonstrates using `.timestamp` to add a `X‑Request‑Time` header. This is
 useful for debugging and tracing requests.
 
-### `.original_url`
+### `.default`
 
 This variable is only available in [dynamic routing expressions](./override_subgraph_urls). It holds
 the default URL for the current subgraph, as declared in your supergraph schema. Because the
-expression must return a URL, you should always provide a fallback using `.original_url` so that
-requests are routed somewhere when no condition matches. The configuration reference emphasises this
-by showing a pattern where the expression returns `.original_url` in the `else` branch.
+expression must return a URL, you should always provide a fallback using `.default` so that requests
+are routed somewhere when no condition matches. The configuration reference emphasises this by
+showing a pattern where the expression returns `.default` in the `else` branch.
+
+### `.default`
+
+This variable is available in [dynamic routing expressions](./override_subgraph_urls) and
+[traffic shaping expressions](./traffic_shaping). It holds the default value for the context in
+which it is used:
+
+- In **dynamic routing**, it holds the default URL for the current subgraph, as declared in your
+  supergraph schema.
+- In **traffic shaping**, it holds the default timeout value set at the global level (available for
+  subgraph overrides).
+
+When writing expressions with conditional logic (like `if` statements), you must ensure a value is
+returned for all paths. You may return any valid value, but `.default` is commonly used as a
+fallback. This allows you to override the configuration for specific conditions while maintaining
+the originally assigned value for all other cases.
 
 ## Available functions
 

packages/web/docs/src/content/router/configuration/override_subgraph_urls.mdx

@@ -59,7 +59,7 @@ decisions.
 Within the `expression`, you have access to two key variables:
 
 - `.request`: The incoming HTTP request object, including its headers.
-- `.original_url`: The original subgraph URL from the supergraph schema, which should be used as a
+- `.default`: The original subgraph URL from the supergraph schema, which should be used as a
   fallback.
 
 ```yaml
@@ -70,6 +70,6 @@ override_subgraph_urls:
         if .request.headers."x-region" == "eu" {
           "https://reviews.eu.api/graphql"
         } else {
-          .original_url
+          .default
         }
 ```

packages/web/docs/src/content/router/guides/dynamic-subgraph-routing.mdx

@@ -22,7 +22,7 @@ For each request, the router evaluates your expression and routes to the URL it
 **What you have access to:**
 
 - `.request` - The incoming HTTP request (headers, method, etc.)
-- `.original_url` - The default URL from your supergraph schema
+- `.default` - The default URL from your supergraph schema
 
 Simple example configuration that routes based on a runtime condition:
 
@@ -34,7 +34,7 @@ override_subgraph_urls:
         if .router.headers."x-use-special-instance" == "true" {
           "https://special-instance.com/graphql"
         } else {
-          .original_url  # Always provide a fallback
+          .default  # Always provide a fallback
         }
 ```
 
@@ -61,7 +61,7 @@ override_subgraph_urls:
         if .request.headers."x-deploy-track" == "canary" {
           "https://products-canary.example.com/graphql"
         } else {
-          .original_url
+          .default
         }
 ```
 
@@ -82,7 +82,7 @@ override_subgraph_urls:
         if random_int(1, 100) <= 10 {
           "https://products-canary.example.com/graphql"
         } else {
-          .original_url
+          .default
         }
 ```
 
@@ -110,6 +110,6 @@ override_subgraph_urls:
           "https://reviews-ap.example.com/graphql"
         } else {
           # Default to primary region
-          .original_url
+          .default
         }
 ```