fix(server): validate every configured client

Signed-off-by: Ting-Hong Shieh <32212900+ting-hong-shieh@users.noreply.github.com>

Author: ting-hong-shieh

crates/switchyard-server/src/config.rs

@@ -119,8 +119,10 @@ impl ServerConfig {
             .map(|name| (name.clone(), Vec::new()))
             .collect::<BTreeMap<String, Vec<ModelConfig>>>();
 
-        for name in self.llm_clients.keys() {
+        // Validate every declared client even when no target currently references it.
+        for (name, client_config) in &self.llm_clients {
             validate_value("llm client name", name)?;
+            build_backend(name, client_config, &BTreeMap::new())?;
         }
         for (target_name, target) in &self.targets {
             let client_config = self.llm_clients.get(&target.llm_client).ok_or_else(|| {
@@ -1132,6 +1134,21 @@ target = "weak"
         }
     }
 
+    #[test]
+    fn rejects_invalid_unreferenced_llm_client() {
+        let invalid = format!(
+            "{VALID_CONFIG}\n\
+             [llm_clients.unused]\n\
+             format = \"openai_chat\"\n\
+             base_url = \"not a url\"\n"
+        );
+        let message = error_message(&invalid);
+        assert!(
+            message.contains("llm client unused base_url"),
+            "unexpected error: {message}"
+        );
+    }
+
     #[test]
     fn an_escalation_table_switches_the_classifier_route_to_escalation() -> ServerResult<()> {
         // Present: the classifier target judges the weak tier's reply each turn instead of