fix(response_cache): don't require redis when disabled and require it when enabled (#8684)

Author: bnjjj

.changesets/fix_response_cache_redis_requirement.md

@@ -0,0 +1,10 @@
+### Validate Redis configuration based on response cache enabled state ([PR #8684](https://github.com/apollographql/router/pull/8684))
+
+Previously, the router would attempt to connect to Redis for response caching regardless of whether response caching was enabled or disabled. This could cause unnecessary connection attempts and configuration errors even when the feature was explicitly disabled.
+
+Now, the router ignores Redis configuration if response caching is disabled.
+If response caching is configured to be _enabled_, Redis configuration is required, and missing Redis configuration will raise an error on startup: 
+
+> Error: you must have a redis configured either for all subgraphs or for subgraph "products"
+
+By [@bnjjj](https://github.com/bnjjj) in https://github.com/apollographql/router/pull/8684